High level swimming requires a lot of practice, over a long period of time.
A highschool swimming program would not have the time or knowledge - to successfully develop elite swimmers.
The best highschool swimmers usually do most of their training elsewhere.
And many of the best teen swimmers never swim a highschool meet.
With swimming there are no tactical races and all courses are more or less the same difficulty.
Recognizing and recruiting talent is pretty simple.
With distance running some kid's will become pretty fast, pretty quickly, with only a little training.
And some kids will luck into a very good highschool program.
Better training, over a longer period would yield better results.
But the US is a large country.
And college running provides a great opportunity for some.