It's an aversion to seeming contrived. Men just fit the trope and archetype better and directors will naturally play it safe. It isn't just the opposite of what a woman "should be", it's the opposite of how a woman generally wounds you and to be frank I'd rather a werewolf tear me limb from limb compared to the malice even the average woman is capable of.

And believe you me a female werewolf can be done, but you also have to remember that the era of the monster movie is kind of over.