It's different between coaches and players, so coaches have nothing to do with a person's career as a football player and there are even some great coaches who are not basically football players.
It's a bias.
There are some great coaches who weren't football players,
but most of good coaches were football players.
It's make sense because you can't feel a sense how a good strategy and team play was supposed to, the way is become the player. Arsene Wenger is one of most popular coaches who wasn't a player.