I think most good HS coaches have learned how to develop runners (or they just have huge teams and it's survival of the fittest!). Coaching less talented people teaches you how to coach, personalize training, etc, if you are invested in making your athletes the best they can be. I really believe that. Most college coaches have not coached developmental athletes- they started as college coaches, and are perhaps not as skilled at tweaking things for kids who don't fit their "system" or kids who come in as lower mileage athletes. I'm always amazed that college coaches recruiting my athletes aren't interested in talking to me about the kid's high school training. I don't know if it's arrogance, or just lack of interest, but to me, it's pretty relevant information to know what the kid has been doing and succeeding with, training-wise.
So many things factor in to college runners not improving. I have a girl who was a Foot Locker finalist who hasn't improved a lick since high school. But she's had 3 different coaches in 3 years at her college. She wasted her time running 800/1500 early on in college when she is a total long distance girl. This was obvious to me, but evidently not to the college coach. It makes me sad, because I see other girls she raced and beat at Foot Locker running at NCAAs, and she can't even run as fast as she did in HS. Sadly, this is a pretty common story.