There is no bigger difference in talent level between college and the pros than in baseball. It's almost hard to watch this stuff sometimes, the quality is so poor.
Most of the big time prospects get drafted out of high school.
That's definitely part of it, but not entirely true. The majority of first rounders this year were college guys.
A couple other reasons: the talent gets spread across the country instead of in a couple powerhouse schools and conferences like in football/basketball, so it thins out quite a bit on every roster. Also basketball and football have lots of guys in the NFL/NBA who still have a lot to learn, and the quality suffers, as anyone who's watched an NFL or NBA team through the rebuilding process can tell you. Baseball doesn't have to do that because they can let players develop in the minors without losing years of control of the player, so the difference between the finished product and the young guys is even bigger.