The real reason.
I heard a movie write once say that for any story to be believable, you can really only expect the audience to believe one totally unbelievable condition. So if you make a movie where someone goes back in time, you have to make the rest of the story fairly realistic. People’s minds can get past one dose of disconnection to reality.
The best games are the ones you can submerge yourself into, and forget you are playing a game. The trick however, is almost every aspect of a game is a disconnection from what can be real. A human can’t really make that jump, or get shot 10 times and still fight. A human cannot carry a 130 pound gun, and run full speed. There are a lot of things in video games that make them more fun, but remove realism.
The more realism you can get back into a game, the closer you can get to people believing it. Fighting men is a dose of reality. Fighting aliens, well, is just another aspect that one must overcome to feel like they are in the game, and not just playing it.







