Not too surprising. This kid of how is was in the 60s, 70s and early 80s. You had H.S. kids running a lot of mileage and volume on the track as well. By the time they entered college, so much repeated stress had been placed on their bodies that they gave away their best years to H.S. However, the ones who survived and made it through that H.S. grind Ryun, Pre, Nelson and Virgin for example went of to become very good as adults. But, there were a lot of athletes like Hunt, Danielson and Pete Richardson for example who pretty much fell off of the map like the guys you list. In running, there has to be an acceptable level of attrition.
During the 90s, young athletes weren't pushing as hard in H.S., and as a result had longer careers. Just about all of you top H.S. kids from the 90s ended up being the top NCAA and pro athletes years later. Then fast forward to the 2000s, athletes like Ritz for example began pushing harder in H.S. as a result you start seeing more young athletes fall by the wayside.