It's not about making it in the NBA
It's about a gauranteed contract for millions.
If you get drafted in the 2nd round, and don't have the skills to make it, you get flushed out.
If you get drafted in the 1st round, whether you have the talent or not, that money is guaranteed.
The caveat for staying in school is getting drafted higher, getting that guaranteed money, and then whether you make it or not, at least you got that ONE contract.