That's a bit sweeping to just lump all the track and field events into one, don't you think? The running events and swimming events can be counted against each other, but suggesting the field events should be lumped in is like counting diving with the swim events (of which there are 4, 2 of which are synchronized events)..and I'll leave out synchronized swimming, since it's something that is performed in the pool (like water polo) rather like a football (soccer) game being played inside a track stadium.
But let's leave out the field events and the multi-events. That's not what is being discussed- no one remotely suggests that there is crossover between throwing an object and sprinting. Sprinting and long jump? Absolutely (Carl Lewis is a prime example for sure). But they're still distinct events- one you must beat your opponents to the line, the other you must jump as far into a pit of sand as possible. I don't think you'll get anyone complaining a field event vs. a track event are the same in principle (I'm not gonna compare between field events, example between triple jump and long jump).
So, direct comparison- pool racing vs. track/road racing. In running/race-walking you have 15 events for the men, 14 for the women. That's already 9 more swimming events than running. The easy distance comparisons (in metres) are:
Running: 100, 200, 400, 800, 1500, 5000, 10000, Marathon, 4x100 relay, 4x400 relay
Swimming: 50, 100, 200, 400, 1500, 10k, 4x100, 4x200 (all freestyle)
More standard distances for running: 9 vs. 7.
The biggest heard complaint with swimming is this: then you add in backstroke, butterfly and breaststroke. The key for the above swimming events is that they are all "free" in how you choose to swim them, but front crawl is the universally chosen method because it is the fastest and most efficient of the four common strokes. Hence front crawl and freestyle have become synonymous with each other.
The distance breakdown for the other strokes in the Olympics is:
Back: 100, 200
Fly: 100, 200
Breaststroke: 100, 200
The common argument is that these are completely separate events, and are akin to hurdles vs. sprints on the track. Here's the flaw I see in that argument:
Hurdles stand in the way to the finish line, but it is the runner's aim to travel at your quickest speed possible as you clear them. The basic way of getting around the track- running- doesn't change, it's just that jumping is now involved. A fairer comparison to hurdles would be putting obstacles in the water and having competitors swim around in the fastest way they could.
The speed factor is the biggest issue to those who oppose multiple swimming strokes being raced. As mentioned above, front crawl is the fastest stroke in the water, and is the one you will see used by those in the 10k distance swim. No one tries to prove who is the fastest swimmer by swimming a stroke that puts them at a disadvantage- do you see triathletes swimming butterfly? The issue is that each of the other three strokes are slower than freestyle, and effectively, it's about who can swim the less efficient stroke quicker than anyone else when medals are awarded in these events.
How did we even end up with four strokes anyhow? Historically, it was simply about who could swim the fastest over a given distance, whichever form worked (freestyle for everything)- this was the case even in the first Olympics in 1896, where all the events are freestyle. For the English in earlier times, the chosen method was breaststroke. But North American Natives who raced the English trounced them with their more efficient form (others around the world swam a front crawl variant as well it should be noted). The English weren't fans of this other style (too much splashing, etc.), but at the first Olympic games, any swimming style could be used. In 1900, backstroke was added. By 1904, breaststroke had earned itself the right to be a separate event. And so 3 of the four events were firmly entrenched in the early years of the Games.
So what about butterfly? In the 30s, it was realized that a more efficient arm movement could be used when doing breaststroke (over the water as opposed to through it). This was seen as a variant of breaststroke until 1956, when it became its own Olympic event and allowed the use of dolphin kick, which isn't permitted in breaststroke (following the historical trend that if a better stroke is invented, then it just becomes another category to race rather than replacing another).
In summary, in terms of speed, from fastest to slowest, the strokes are front crawl, butterfly, backstroke, and breaststroke. Each requires different techniques, and so the fastest swimmers in one event are not always the fastest swimmers in others (but great swimmers, such as Michael Phelps, can succeed across all disciplines, but of course not all distances).
Alright, what about race walking then? It's a clear instance of purposely going slower towards the finish by not allowing both feet off the ground at the same time. There are even penalties given out during the race if a racer is caught with both heels off the ground (3 strikes and you're out). For all of these reasons, race walking and the lesser strokes in swimming represent a fair comparison, except that cheating in while swimming a certain stroke is grounds for an automatic DQ, while some race walkers will try to play their advantage and have both feet off the ground when no one is looking, and get away with it (watch a slow motion video of a championship walk if you doubt this). Why is race walking in the Olympic? Again, historical reasons that placed it and running as separate and distinct events, even if the common goal is the same.
So if race walking awards medals, what's with all the fuss above too many medals being handed out in the pool? The obvious response is that there are two race walking events for the men and one for the women currently, while there are six for both the men and women of the lesser strokes. But the real wrench thrown into the running vs. swimming debate is that swimming awards medals in the 200 and 400 individual medleys, and has a 4x100 medley relay.
Medleys consist of all four strokes (on the relay, each individual swims one of the four strokes), and recognize who has mastered each of the four strokes best in the individual races. Michael Phelps and Ryan Lochte are the world record holders and winners in these events, are so are often referred to as the best all-around swimmers in the world. What helps their case in addition to this is that Phelps and Lochte are among the fastest in one of the freestyle events (200), and swim on the 4x100 freestyle squad. There is no question they are among the fastest swimmers in the world, even leaving out the slower strokes.
But compare this to a pure running event- imagine running and race walking combined in an event, and you have the medley. What about the steeplechase? Again, a competitor is not changing their running form on the flats- the goal is still to run as efficiently as possible to the finish. The challenge is running as efficiency as possible while having enough energy to hop the barriers, all with somewhat wet feet after the water jump.
A comparison could be made between the decathlon/heptathlon and the medley. Again, if you're lumping throwing and jumping events in with running, you might as well ask where the diving is for a swim medley. The decathlon does not contain a race walk component (though it did at one time). There is no comparison.
And hence someone asks, "if I run backwards better than anyone else, is that not akin to swimming backstroke and getting a medal for it?" And if the IAAF thought backwards running was common enough to hold competitions, it likely would be. After all, the skill set required to hop, skip and jump was considered different in a useful manner from the ability to simply run and jump as far as possible, and so we have two horizontal jumping events which crown different champions. Throwing different objects has historical context in warfare, and so we have different throwing events, (with little crossover between champions across events at the Olympic level it should be noted...again different from swimming).
But getting back to running. Simply put, people are not complaining there are too many freestyle events in the pool. It's the other events that are the source of complaint, for the same reason race walking takes flack: awarding medals to those who get to the line the fastest in an event that dictates racing at a speed below your fast horizontal limit.
As to who is the greatest Olympian ever...it's silly really. Let's only compare competitors to there contemporaries in their respective sports. Phelps is the best swimmer ever. Enough said.