They have a responsibility to develop art assets that are in demand by the community. Female avatars which are not attractive are, historically, not in demand by MMO players (surprise!). When you're a game developer trying to decide how an artist should spend his time, you have to consider what the majority of players want and will be interested in. Ugly female character models are not among the list of things that the majority of MMO players want.
It's just not practical for them to spend a bunch of time and money developing art and models that a tiny handful of players might use, particularly when most players at this point have already maxed out their available character slots and so new characters are relatively few and far between.
This is not about equality. It's about the fact that art is a time-consuming process, and almost nobody is asking for more female character models (including female players, who also are historically uninterested in playing ugly character models). I respect the fact that you want this, but it's important to understand how much in the minority your opinion is.