A couple more points:
The reason some young players don't seem "ready" for the NBA is because the way the draft system works, the teams drafting the best players (and youngest, since the top prospects usually leave early) are the worst teams. This thrusts the player into playing significant minutes immediately because:
1) The team doesn't have anyone better.
2) Rebuilding teams are selling their fan base on hope - there is a lot of pressure to play the young potential star early.
3) Since they're not going to be any good anyway, letting the young player accumulate experience and "learn the hard way" doesn't cost them anything, in fact it likely helps them tank.
This is why the draft system must be changed if we're going to help solve this issue, if you even consider it an issue (which I don't). People never bring up the players who go to the good teams in the second half of the draft when talking about "unreadiness" because the good teams don't need their draft picks to contribute right away.
Regarding the NBDL, it's still a very imperfect thing. Each NBA team doesn't own and run their own NBDL team, so you're giving away your player and having no control over him. The system, training, instruction, might all be different from what your organization teaches.
Not to mention basketball is different from baseball in that it's a very team-driven sport. In baseball it's harder to be selfish because usually what's good for your stats is also what's good for the team. The NBDL is basically a league of players trying to put up numbers to attract an NBA team, and chasing stats to the detriment of team play is common. It's not an ideal situation for developing your players, but there isn't any better option right now.