A dev's p.o.v
Hi,
I work for one of the major game developers. I'm totally in favour of greater female representation; Christ we need it. Here, though, are some reasons why female characters rarely figure.
Modelling and texturing geometry is relatively cheap; that's not the issue. What makes it way more expensive is that you need to create a new skeleton hierarchy to drive that geo. That skeleton has different pivot points for narrower shoulders etc without which your female characters would look like a bad tranny.
Once you introduce a new skeleton type into a game there is a phenomenal amount of cost associated with that. If you're sensible all the animations in the game will play on that new skeleton, but there will be major issues such as hands not holding guns, since the bones are different lengths, or heels not behaving well with the ground plane. A large number of real time systems need to be used to correct these issues, but it gets pretty complicated pretty fast.
You will probably need complete replacements of animation sets for movement, climbing etc. Some systems such as hand to hand combat might need to be designed to cope with a female grappling a male and vice versa. The last game I worked on had over 25,000 animations. Each one needs to be lovingly authored to work with each other.
All of this costs a lot of money, which results in one decision being made, and here's the crux, usually late in the day. Do we drop the ability to do 'X' even though it's really fun, so we can support female variations/interactions for Y and Z? That decision frequently goes in favour of dropping female variations altogether, since people who make games usually make them for one reason above all others. To make fun games. And fun beats diversity nine times out of ten in the world of games production, which mostly is made up on the hoof. That last point is worth underlining.
There are some devs out there that really understand the power of strong female characters. Look to something like The Last of Us by Naughty Dog for an example of a stunning game with compelling male and female characters. To achieve that takes meticulous planning from the outset, which I can tell you rarely happens. And this is the real problem behind all of this; shoddy planning, not sexism.