I think it’s a simple calculus: if you are certain to be drafted in the NBA draft, and likely in the first round, you look at the salary and contract and say, “that looks like a pretty good career opportunity.”
He can always go back and get his degree. But the opportunity to be drafted and become an NBA player – no matter what team it is – is too good to pass up.
I try to imagine if it was in another field, like engineering, or business, etc. If Google/Tesla/Amazon contacted a freshmen engineering student and said "come work for us and we’ll pay you X million for the next 3 years with the opportunity to go up from there… Most would take that.