I think people are motivated to run competitively if they are still improving after college. The idea of paying for races, waking up at 5am to run them, and dedicating time before/after work and on weekends to train for them would be much less appealing, at least for me, if I weren't going after strong performances and PRs. I didn't run particularly well in college (ended up quitting after my sophomore year), but after a few years off I felt I had unfinished business. I've since dropped my 5k from low-16s to 15:06 and run PRs at most other distances. I'm just guessing, but if my college career went really well, I'd probably just run a few days a week to not get fat.
Another variable to consider; longer-distance runners are probably more likely to keep going. There aren't very many opportunities for non-pro milers, but there's a whole new world of 5ks-marathons and ultras to get into.