They make all the money from TV deals.
All the NFL players have to do is to go on strike until they get what they want.
If they were willing to accommodate FIFA, they can do the same with the NFL players.
But the NFLPA is weak compared to the MLBPA.
Accomodating FIFA in June and July, for a low surface impact sport such as soccer, is not the same as maintaining grass in September through December for football.
No one in soccer is forming up 300 pound linemen, digging into the ground to push through a defensive line, which is also dug in to stop them. And then repeating it less than two minutes later, continuing to do so for an entire game and possibly overtime.