The answer to this apparent conundrum is really pretty simple:
Speed.
Everyone in the NFL is almost without fail BIG and fast, fast fast (aside from kickers, punters, and pocket QBs, and some smaller skill guys who are nevertheless abnormally fast.) Speed is a prerequisite, a barrier to entry. You rarely see the speed and size mismatches in the NFL, so you see far fewer huge plays. Every once in a long while you get some freakish confluence of talent (like the Greatest Show On Turf) where risk is rendered almost null but it's a rarity.
In college, you find those mismatches quite often, which compels you to take risks to enable scoring. Unless you're Iowa.