Obviously a lot of them are, but there might be a little bit of a selection bias going on here.
There were 63 early entries for the draft last year; there are only 60 draft picks, plus that doesn't include college seniors or any international players. There are a lot of early entries who don't get drafted, or go late second round, every year. I'm just scanning the list from last year, even guys from "big schools", Isiah Briscoe for UK, Melo Trimble from maryland, DJ Wilson from Michigan, Kobi Simmons from Arizona. Sometimes guys are just done with college.