Who Knows.
Maybe they like to have a different challange then player vs player.
Bots is stupid or aimboters.
So you are like. " Ah, I want to pawn some zombies. Lets set them do noobs"
So in this way, a player will feel better about their skills.
It can also be like, "I hate all the players in mw2. They are all fu** cheating"
And this is more like a anti-cheat way to play. ( if you get me.. )
And I guess, many people like zombies.
they are cool, dead, wird, they have no GUNES.....
But yeh, it can become boring very fast.
I hope you got some answers on maybe why