system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: why do so many athletes who were great in college fail at the professional level?

It would stand to reason that if you're the best quarterback or point guard at the college level, you'd go on to be the best of your draft class in the NFL or NBA.  But often it just doesn't work out that way - aside from the obvious train wrecks like Vince Young, JaMarcus Russell and Ryan Leaf, you also have the likes of Joey Harrington and Heath Shuler, who don't appear to have any character issues but just couldn't get it done at a professional level.  (Meanwhile, seemingly marginal players like Tom Brady or Kurt Warner go on to tremendous success.)  Why is it so hard to transition from college to the pros?
gpt
At the college level there is likely to be only 1 or 2 draftable players per team.  At the NFL every player was good enough to make it into the NFL.  It's much easier to be a lone superstar at the college level.  For the NFL football is MUCH more a team sport.  Players frequently get into trouble in the NFL when they arrive thinking they are going to be the superstar on the team just like they were in high school and in college.  

Frequently those players adapt and play well in the NFL.  Sometimes they don't adapt that well and are not successful at the pro level. Playing at the pro level uses many of the same skills as the college level but it's not exactly the same.