1.) Your best performance comes when you're about 26 years old, NOT when you're 18.
--Although this can be true many who continue to train hard after high school do not run appreciably faster.
2.) Most people in HS have crappy coaches. It doesn't matter whether you're dedicated or not, chances are, you won't go anywhere if your coach is making you do stupid runs (intervals 5 times a week, for example).
--At the very high levels this would be true but solid training by those who try, it shouldn't matter much if 5 is so easy.
3.) Guys in the military have the least capable body type for distance running. The military tends to attract guys who are good in football and sprints, not in XC. Just count the number of guys who weigh under 130 lbs and compare it with those over 200.
--NOt suited for distance but still more suited than the average person, these are for the most part high achievers and cannot be seen to be bringing down the average. As a group, they would be not as good as pure runners but still much above average joe.
4.) The original question was getting to a sub 5 under ideal training. Anyone who says "sub 5 is too hard, only 2 guys in my HS got it" is not taking the original question into consideration. Ideal training is not "work your ass off while you're 14-17 and hope to get a sub 5 before you graduate", it's "do a lot of easy runs since you were in third grade, live at altitude, continue to run after college, and get a sub 5 once when you're around 24-28 years old."
---Altitude would help slide down the guy who trains his butt off but only gets to say a 5:10. THe starting point is where the discrepancey and that's what base and average MV02 levels addressed; the training effect can only do so much.
5.) Stories that tell of people not even close to breaking 5 after much effort are exceptions. For every guy who can't break 5:30 with a lot of training, there's another guy who's running 4:30's. Average everything out, and you'll probably be around 5 for the average guy.
---THis is the biggest leap; you are still comparing people who are runners; they pursued it because they had talent, albeit different levels. I would agree of the guys who are predisposed to pursue running; 50% can eventually break 5 under the ideal circumstances; equipment, coaching, etc. The original question is 'guys in total'.
No one here it is hard per se. I'm saying it's out of reach of more than 90% of the group we are talking about.
Purely on gut feel;
Another way to look at it is this;
the world record is 3:43; 16% slower is a 4:19 mile or 4:00 1500. ANother 16% puts you at a 5:00 mile. It is inconceivable that the average person could be as close to a 4:19 miler as the 4:19 miler is to the world record holder! Average person. Not a subset of athletes etc. I think as athletes some runners seem to figure that everyone has a sport even if not running. IT's simply not true! THere are a lot of people not predisposed to any activity.
(sorry I don't know how to do the quotes thing to make it easier to read)