Everything. The CBA in football revolves around the very real threat of injuries in football and do everything they can to limit snaps. That's why the 15 minute to 10 minute overtime length change, which was stupid, was passed in 2017. That's why overtimes were taken out of preseason. That's why special teams rules seem to get tweaked every other year. It's even worse in college where people aren't making money and basically playing multiple OT's worth of extra snaps for pride and for other people's entertainment, particularly the vast majority of college players that have no shot to be drafted and make the NFL. In the NFL, the players' union would never go for it, and rightfully so.
Baseball's unlimited extra innings is stupid even if you're a hardcore baseball fan and don't mind staying up until 1AM to watch 15+ inning games, but it doesn't take a toll on the players' health and fatigue the way that extra snaps do in football. Most of the time baseball players are standing and squatting waiting for a ball to come to them or in the dugout. Sure, pitchers' arms get tired, but that's what a bullpen is for. In football you don't have 100 extra players to put on the field in such cases, and if you did the financials just wouldn't work.