Lazy developers who keep creating WWII games give the impression that people enjoy shooting germans/humans.

Now our generation of the "Axis of Evil" is the terrorist, so games are shifting from germans to middle eastern men.

But look at past games like DOOM, DukeNukem, and Quake.. Three of the most popular games where all based on shooting monsters. You can add Halo to the list also.

I think developers stop making monster games because the market was oversaturated with.... monsters. Now it seems that the market is oversaturated with WWII/III games.

Personally, I don't think people care what they shoot, as long as its fun and challenging.

Kitties.  They're far far more dangerous.

I don't like aliens, they freak me out a bit. Also, you can take out your anger much better if you are shooting humans, also you can relate to the story if you are shooting evil terrorists (can you????)

I don't think about killing people when playing such games.

Those games are more about tactics and aiming skill, trying to best the other players...the fact that their characters die doesn't usually cross my mind. And it shouldn't, since they're not really dying, they're just "knocked out" until they respawn.

We Europeans love shooting Americans every chance we get in games.

As for why people, because human beings lust for blood.

Looks good... wouldn't like to be that dog...

It's not that I like shooting humans so much as I like shooting everything. Allies, enemies, aliens, koopas, bugs, whatever, it's all good. If they get in my way, there's at least a 101% chance that they will die sooner or later.

As long as it falls down dynamically and twitches slightly after getting shot, allowing me to run up to it for the obligatory teabagging session afterward, I'm probably enjoying the game on some caveman "me kill, you die" level.

With that said, I'm a really nice guy in real life with very few plans of shooting anyone in the near future.

because the only real enemy humans have are humans

Same, although I try not to play games as if it's a game.  As in, I try to play cautiously, as if my life weren't expendable and I would just respawn if I lost.  

because they like it and they think it's fun.