Actually the NBA does have a farm system, the D-league. They just don’t tend to use if very much (few players promote from the D-league to the NBA). The NFL tried having a farm system with NFL-Europe but it failed as a league, they do pull players out of the arena league, they could tighten their ties to that.
That is nowhere near as robust as MLB’s farm system. I think they would develop better players if they set one up. Heck, that is why they bring in so many foreign players, they come through programs that emphasize skill development over time.