From an admin and funding standpoint, there would probably be substantial benefit to bringing as much as possible under the NBA umbrella. But I doubt the NBA wants to get bogged down with non-elite junior and grassroots competition. While the NBA academies, NBA junior world championships, and the NBA's increased involvement in Team USA junior sides is a step in that direction, all of that is aimed at increasing the NBA talent pool.
Given the value of franchises, there would be a lot of pushback from owners against significant expansion that devalued their team. Russian oligarchs probably don't want to buy the Brooklyn Nets if they can buy the Moscow Donkeys...
And ultimately, I agree with Hoopie that the NBA's best interests are not necessarily basketball's best interests, and keeping the two separate would be desirable. However, FIBA is such a lightweight compared to the NBA, that it's hard to see them ever being the genuine driving force behind international basketball.