The best players have sooooo much fucking money. They just don’t need the money or stress of coaching, probably just happy to retire. You’re more likely to find backups and journeymen become coaches later, because the money is actually still life changing for them.
Positions are so specialized, just because you’re good at rushing the passer doesn’t mean you can coach a whole team, or even just the defense. You don’t really have to have knowledge of the whole field. So playing well doesn’t necessarily translate to coaching well.
You might be surprised by the number of players that really just don’t care about football all that much. They’re there to be good enough to make a boat load of money, then they check out and never want to be involved in football again.
You just had to jinx it, didn’t you?