I'm a mediocre talented runner. My 5k PR is low 15s (at elevation, I never raced a decent sea-level 5k). So, with a time in the low 16s, I can win the majority of hobby jogger 5k races. But I rarely run a 5k unless I know there is prize money in it. If there is no prize money, showing up and easily winning a hobby-jogger 5k is just a waste of time and money.
Case in point, I ran a 5k this last weekend just for the sake of running a race since the last time I raced was in the fall. The race was $27.50 to sign up for. I won. I was done running at about 7:45 (race started at 7:30). Afterward they gave away a ton of stuff in raffle prizes. They did a fairly good job of quickly giving the raffle prizes away because they posted the numbers on a big poster board. There were about 500 people in the race, and they gave away about 150 things and some of it was pretty good stuff like sleeping bags or 4-man tents. I didn't get anything. No problem. Maybe with such good raffle prizes, they will give something decent away for the race prizes.
The raffle prizes where all given away by 8:30. It's starting to really warm up and standing around in the sun is getting a bit annoying. The last 5k joggers are walking in. There are two big boxes with "race prize mugs" sitting on a table under the a tent. A few people ask a volunteer who seems to be running things, "so when are the race prizes going to be given away?" He says, "I'm not sure, I'm not the race director, but just 1 minute." 10 more minutes goes by. Nothing is happening. More people ask when results will be announced. Again, "just a minute." Another 10 minutes goes by. The laptop computer on the table next to the finish line is just sitting there, closed. The inflatable archway over the finish line is now being dismantled. I ask a few volunteers "so is it worth waiting around for the race prizes, what are they?" Answer "we don't know, but it will just be 1 minute. But if you get a ribbon, we can mail it to you." I hesitate for another 5 minutes and decide it's time to leave.
So, unless it is an established well-run race, this is a very typical race experience. I could repeat at least 2 dozen stories with this sort of experience that I've had. If I was just a hobby-jogger (I know some on letsrun will say my 5k pr makes me a hobbyjogger, let's set that debate aside) and I ran a 5k on a Saturday morning, it would be a fine social event and I would leave right after I'm done. But when you are the winner or another top finisher, you kind of want to stick around just on the off chance they will give you some nice swag. I tend to try one of these races about once a year hoping it will be well organized, and 90% of the time I'm disappointed and reminded why I don't want to run any races I don't already know are well organized.
So the bottom line is that unless it is one of the few races in a given year that you know is run well, and gives away decent stuff as prizes (not just as raffles), there is no reason to show up. Doing your own solo 5k TT on a Satruday morning is more enjoyable than going to all the trouble of running a race.
Races that are well organized and can be relied upon to give decent prize money will attract the post collegiate talent that will show up and win the thing in sub 15. Otherwise, the decent runners have learned to not bother.