Roy and Tyler were ALREADY PROJECTED to the first round the prior year. Tyler was rated around 20-22nd his final 3 seasons (which is why I say Bird reached on him, he was being mocked at 20th).
Roy actually FELL in his draft standing.
They both stayed because of the program, their belief in it, belief in the coach, education, etc.
Even Blake Griffin took a risk simply to move from 6-7th to 1st, which was in no way certain going into his sophmore year. He improved, he bet on himself and won. Had he remained the same it likely would have lowered his stock because the feeling becomes (as it was with Roy and Tyler) that they've tapped out their potential and hit their ceiling.
I like kids that stay in school longer because you learn the sport more and it tends to show some level of character. Frankly I think Jennings showed character by manning up and taking on a challenge even bigger than college, which is to move to a foreign country completely and then sticking with it even when things were rough for him.
And as was said, there's more to life than money. What price does an 80 year old man put on one more year of being 21 and in college? I'll bet it's more than a couple million dollars.
Staying in school can make your career better or it can hurt your draft stock. It's an unknown factor. There are pros and cons to it and not just a few of them.