More or less it's because exclusive games are designed around taking advantage of the hardware. First you have a whole team dedicated to making sure the code is efficient and gets the most out of the hardware, then the other part you have games like Uncharted and God of War which are linear that makes it much easier to use more resources, than an open game like inFamous, GTAIV, or the recent Red Dead Redemption.
Multiplatform games are multiplatform to get the most money, making teams designed to get the most out of porting the game to specific consoles would cost more than just a general team that can make the game run on both consoles at a satisfactory rate, Infinity Ward was one of the few teams I can list off that would make a dedicated team for each platform so their games would run the same no matter what, but they're an exception to the rule plus had the means to do so unlike many other companies.
MaxwellGT2000 - "Does the amount of times you beat it count towards how hardcore you are?"
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