Oh, they tried, they tried.
Remember Deus Ex: Invisible War?
It had the exact same coding for Xbox and PC; but while this caused a great game for X-Box standerds, it was crap for PC standerds, and also gave loads of framerate issues for many PC users due to crappy X-Box optimisation...
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I mean like with an emulator. Stick the Xbox disc in your drive and play it on the PC.
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I would suspect that the code is not robust enough to worry about variable CPU speeds, memory, video card, etc.
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Hence the emulator. I mean SNES, NES, Genesis, PSX, etc. games weren't even programmed for a Windows-type OS, yet you can play them on your PC with an emulator. You'd think that a game programmed for a simplified windows OS would be easier to program a windows based emulator for.