Baseball (MLB) has a minor league, Hockey (NHL) has the AHL, Basketball (NBA) has the D-League, and even Soccer (MLS) has the USL and NASL although they may not be farm leagues in the same sense as the other sports.
It could be argued that NCAA college football is a developmental league, but that alone does not explain why there is no professional developmental league for after college since all the other sports listed above have NCAA college leagues and still have a farm league.
