The NHL and MBA maintain farm systems. The NBA and NFL do not. That is part of the problem. The NBA and NFL don’t want the cost of farm systems. Colleges want to pay their superstar athletes a pittance. So they feed off each other’s sanctimony.
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.