Sex sells. That's pretty much it. Characters are made to make the average gamer like the game more, in a lot of series. Most gamers are guys, so they target men. This generally means the women have to arouse us in some way, curvy arse, big breasts, narrow waist, etc. This is why the men in games all have biceps larger than their thighs, we see them as "cool" and Strong", and since we usually play them, we are therefore "strong" and "cool."
The fact is, despite the fact that people are saying "Women are more than their breasts" the fact is since most men don't think that way, they've pretty much cornered the market. That's why most girls in games are stick-thin with strangely large breasts for their size, and yet they can run, jump, backflip and vault over objects without ripping their own chesticles off.
When people stop liking breasts so much, women will have more than breasts. Until then, we're stuck with Lara's, and Lulu's.