Because they are rare?
Because some gamers have low social skills, and little understanding of how to treat the opposite sex?
Because there is a culture of "using" women online - that they are there only for the men's satisfaction? I'm not exaggerating, btw, I can't go on any forums other than here or Facebook without being harassed to "get my tits out" by every male in the forum.
Maybe it's these experiences that have made sure that I have never gamed online, only through LAN games with people I know. I don't feel good when I'm treated like a sex play-thing when I actually want to play video games. While I try not to get pissed off, cause I can just imagine them all sitting in their empty homes, too scared to go outside and talk to a real woman, it's still sort of scary.