Because they wanted in on the console profit but were too lazy/stingy to develop separately for the PC. So they give the PC gamers a game that doesn't exceed the capability of the consoles (level size, AI complexity, texture resolution, etc), looks like crap but market the hell out of it, and hit three birds/platforms with one stone.
The options are:
Develop mediocre game for the PC, make $$.
Develop fantastic game for the PC, make $$$
Develop mediocre game for the Xbox/PS3, make $$$
Develop fantastic game for the Xbox/PS3, make $$$$
Develop fantastic game for the PC, port it to consoles, make $$$$$
Develop game for Xbox/PS3, port it to PC poorly and buy a few reviews, make $$$$$$$$$$$$$
They took the biggest payout. Simple. Problem is that last option winds up burning a good portion of your customer base every time. So it's questionable how many times you can repeat it. Although CODMW2->CODBLOPS managed to do it without much of an issue.






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