Because back in the day when blatant sexism was the norm, women were excluded. Women-only tournaments were an attempt to correct that.

In a perfect world, everybody would play in the same tournaments and all genders would feel equally welcome to them, but that's not the world we live in.

The men’s tournaments are open to everyone and women have and do compete in them.

There are open tournaments and women's tournaments. The open tournaments are for anyone. Women have competed in them but at the top level it is somewhat rare.

The women's tournaments are intended to provide an environment that encourages more women to play chess. There's considerable sexism among men (not only in the chess world, hah) which is not always very welcoming.