Because it's a sexist conception *no pun intended)
If a couple of girel musicians form a group. and hire the best musicians they can find, and it turns out that they are all women, that's ok.
But (no pun intnded) if they decide, before hand, that they'll only hire women, they might just be hiring musicians who are not the best available, and that's not only sexist, but stupid as well, and they deserve to be less than excellent.

And the same goes for men

So there

Jody Gibson.