Mcoffey said:
Because modeling is hard.
This, this, and MORE OF THIS!!!
I've tried modeling for a 2d game, not even 3d, and it was extremely hard. Add in the third dimension and it becomes very difficult to make a decent-looking female. No offense, but men are generally easy to model in a game because you don't have the... Well features to add in and such.
I also think it probably clashed with the engine somewhat probably.
Though, on the note of for instance Turians, what's to say you haven't? I mean, the majority of Turians have a militaristic outlook, and generally, a buff woman's features don't stand out nearly as much. With this in mind, a female Turian could look extremely similar to a male turian.
There's no way to tell if there is more than one race of Hanar, they could easily be Asexual.
Volus are covered by a suit, again, no way to tell if there's been a female one or not
Elcor... They speak in monotone voices, what's to say that a female Elcor doesn't look like a regular elcor?
I think for the most part there's no easy way to tell, for the majority of species, if there has been more than one gender seen. Those with obvious physical differences though, are likely due to modeling restraints.