It’s all money. Football is the most expensive sport to play and semi pro teams don’t last long. They only lasted a few seasons at best.
Plus the violent nature of football means that it’s more likely a player is going to be so injured that they’ll never be able to get to the level needed to make it to the NFL. It’s not like baseball or even basketball in that respect, the depreciation of a player in football over time is pretty drastic. The average career of an NFL running back is about four years.