I was a high school athlete in the late 1980s to early 1990s and then coached from 1993 to about 2000.
What did we do "wrong"? (I coached state champions, including a 9:30 3200 runner in a small school championship, and I'll honestly say that we did not optimize outcomes).
Too little mileage. Not enough strength work. Far too much static stretching and not enough dynamic movements and form drilling.
A fear that if anyone got injured it was surely "overuse" and you were a "bad coach." Better to get less out of the kids than to risk getting close to the edge. (I don't know that that is the worst philosophy with kids, but if you want sub-4:00 guys, then risk is involved.)
Too many low-stakes races that interfered with good training blocks. Not enough national-class races to put the best against the best.
In too many of those low-stakes meets the top guys would run for points (4X800, 1600, 3200), and do it again three days later. (I never had my guys run all that crap, but a lot of coaches did).
Kids thought locally. If you can beat the guys around here, that was good enough. With social media, YouTube, etc., the kids have a better idea of what it means to be truly great. It also makes them more aware of what is possible.
We lost a lot of potentially great kids to soccer.
Junk miles were the name of the game in the summer for a lot of kids. Run 500 miles (all slow) and get a "500 Mile Club" t-shirt, without even running some strides.
I could probably go on. Not all of these points applied to every program, but some points apply to most at the time.