Why is it divided by sex?

I can only speak about my nation’s club football (the world kind, not the us one): the normal competition is open to anyone, with smaller competitions for other groups. Those smaller competitions have discriminatory rules for entry, but players that meet those criteria, can still chose to play in the normal competition if they want to. The “normal” competition has many more brackets than the smaller competitions because there’s just way more players, which also means that if you want to play vs the best, that’s where they are. It’s the same principle for all ages.

I can imagine that at one point the football competition in my country had similar “no girls allowed” rules, but when I grew up in the nineties, the football competition that I played in was already mixed.