Always ? I used to play almost exclusively female protagonist, AKA corruption games, I have never seen futa but poor ladies being fucked left and right by everything that moves. Sort of a female forced harem. I don't like futa, so I ignore them mostly, but I have never had problems into finding female PoV games to play.
And of course there is futa on men as well. I could agree that maybe on less proportion since it involves femdom and that is not a community favorite.
Now if I had to analyze the reason why female protagonists are fucked by everything that moves, human or not is because the objective of the game is not to fuck, which tends to be in male PoV, but to defile the poor lady. If you check on sissification games you'll probably find the same trend as in female protagonist corruption games since they share similar objectives.