Because that's fekin' weird. Even for our tastes. Furries and monster girls are one thing, but too fuck a proper female animal is different. Male versions of real animals have human like genitalia and considering the many ways women get of, not that surprising that they'd go after a real animal. For males it's a no go
Hmmm. I disagree on both counts. The most commonly seen male animal on female human (barring tentacles/mythical creatures) in games are the dog, horse and pig.
While I will give you that female dogs genitals do not look much like human ones, the lack of a knot/body sheathe and addition of a glans at the end of a human penis shows that male dogs and humans are just different as the females.
Regarding horses and pigs however, the females genitals are MUCH more similar to those of human females. Human males however do not have the flared tip or body sheathes of horses, nor do they have the strangely thin and almost tendril like penis of pigs.
On the second part, I agree that women get off in more ways physically than men. But for (most) men to get off only one thing is usually needed. A hole. In terms of psychological aspects of getting off, I would argue it is a case by case basis.
The crux of the matter is that animals mate entirely differently than from humans. It is entirely for reproductive or lustful purposes. There is no love. This usually in most societal viewpoints of humans can be seen in caveman like men of old forcing themselves on women in an violent act of getting off and not caring if their partner does. When an animal does this to a woman, it can be excused because it has no cognitive thought and is just acting according to its nature and is enjoying it. But if a man does this to an animal, it looks like an animal abuser using the trust of a creature for its own sexual pleasure, ignoring if the female animal enjoys it or not.
Tl:dr version: Someone make a game where I can fuck a female dragon to make dragonborns a la DnD.