Because the females have better clothing options in general (PARTICULARLY from Granzella)
Because the females have better poses and emotes (twirl > spin. That is all)
Because maybe they're trying to replicate a character's look on Home. (See: Sheva or Lightning)
How many more answers you want? This topic seems to come up at least once a month, and it gets the same responses every. Single. Time. And if you have a problem with guys using girl avvys (or vice versa, for that matter), then... no, I better stop there.
*Lockwood Gift Machine Wishlist (updated 1/31): Blaster's Bay, Whimsy Hollow, Iron Fusion Vault (either gender), Drey VIP Room (either gender) * (PS: Blank Friend Requests WILL be deleted!)