There's your reason why.
Seriously, almost every single woman I see in a video game is either highly sexualized, or gorgeous. It's normally incredibly unrealistic either way, considering, say, if a game is a Science Fiction space exploration Adventure/Action where you fight people/aliens in ships as well as using artillery and hand to hand combat. Let's call it Mortal Kombat Meets Metroid. You'd expect everyone, to wear something that protects them, like a space suit or armor like the kind that Samus Aran uses. Well, they usually do that for the men, but not the women. Instead, let's put the women in armored bikinis instead. It's certainly nice to look at, but when you get into a battle with Ridley or Shao Khan, you'd want to have something on to protect you. It's no wonder women are usually the weaker characters in video games. Well, that and the ridiculous social stigma that women are naturally weak and fragile creatures. They obviously haven't met Joanie Laurer.
Former Intercontinental Champion; can bench press 365 lbs.
And we can't forget the breast physics thing. Sure, the game makers make boobs bouncy and jiggly and all that, but they don't put in other breast physics, such as back pain, difficulty breathing, skin irritation, and difficulty buying clothing. Well, that last one isn't quite a physical thing like the rest, but you get my point.
Just like the Energizer bunny, they can go all day.
And speaking of Samus, let me just say that I idolized her when I was growing up, since she was a tough as nails woman who, admittedly always wore a bikini under her armor, but she still wasn't highly sexualized . . . until she started wearing what's called a Zero Suit. It's a skin tight blue outfit that she now wears under her armor. Granted, it covers more than a bikini, but the problem is that, ever since that switch, Nintendo has been sexualizing her more often. Thanks, Nintendo, for ruining my idol for me. Don't get me wrong, I'm still all for Nintendo Strip Poker, but I don't care for random ass shots just for the Hell of it (even though I'm a lesbian).
Oh, and they've been sexualizing Princess Daisy, too . . .
You know, for kids!
You know what, I may be a lesbian, but most women aren't. Most women aren't going to care if you can see Princess Peach's underwear in Super Smash Bros., and they most likely won't even play a game with a woman in a deliberately skimpy outfit on the cover. They especially wouldn't be touching a game like Dead or Alive Xtreme Beach Volleyball. Well, unless they were like me, but I doubt it, don't you? Hell, even I didn't care to play that game.
Perfect game to show your girlfriend.
Besides the sexualization, there's also another factor that deals with a different kind of mentality, one I've dealt with since childhood. It seems a lot of boys I knew growing up (or at least met in a pizza place somewhere) were usually surprised or even upset with the fact that I was (and still am) a gamer. I liked playing games just like they did, but sometimes would almost be excluded from playing games just because I'm female. There were more boys that I knew who weren't like that than the ones who were, but that kind of mentality was prevalent through most of my life until adulthood and a rise in the population of female gamers. Overall, that kind of mentality seems to be dying down, and it really should, because it's freedom limiting behavior.
Just like you. Live with it, she may be your only chance of getting laid.
Regardless of all of that crap, I'm the kind of person who plays games to play games as opposed to having any other sort of thing, like eye candy, get in my way. Hell, if I really wanted eye candy that badly, I have somebody for that already, and unlike some of the video games that I play, I can never get sick of her.