But man, why is it that so many of the people—particularly women—that are theoretically on our side still suck?

Because they're sociopaths. The only thing they care about is the personal gain they can extract from the situation.

This woman is doing the conflict resolution version of monkey branching. She's currently on side A, but she'll never commit and always try to keep the door open to switch sides and stab you in the back should she at any point conclude that doing so would be to her benefit.