I think often the reason is because it's sometimes hard to get your player character to look exactly like you so it's like, what the hell, might as well just pick something totally opposite. I know in Fallout 4 I found it frustrating because of the voiced protagonist, like trying to make a character look like me but then sound totally different : o wasn't worth bothering. Just jumped to the opposite sex.
Also for certain action games (not to exaggerate a stereotype but just a reality of genetics) it can be fun as a male to play as a female character and like destroy monsters and things. Girl power so to speak. In my brain something like Fallout pops in my head and putting together a supposedly girl next door looking character who's blowing the brains out of Super Mutants. I'm literally the last person on the planet to care about progressive checklists in entertainment, but it is a fact that the majority of video games have male leading protagonists (Uncharted sort of stuff), so it can be kind of refreshing when you can pick to pick a unique female lead. Again, I don't really care generally but just saying.
Anyway I really do think a big part of it is that in a lot of games it's difficult to really make yourself a believable character so people just say screw it. If games like Fallout allowed you to play as different species (like ghouls) I imagine there's be a load of people who picked that too. Mixing it up