MLB draft already is.
I don't get why all sports don't do what baseball does. Basketball used to do something similar, and then reverted.
I would imagine that their approach has a lot to do with how it's remained a true amateur collegiate athletic sport.
After high school, you make a choice. You either think you're hot shit enough to forgo college and skip straight ahead to the big leagues, or you go get your education. But once you've made that choice, if you chose pro you can't play at a college level, and if you chose college you're stuck with it for at least 3 years.
Makes too much sense for football, I guess.