"How many people buy Macs to run Windows games" is the wrong question.
I don't buy computers just to read websites (I can do that more comfortably on an iPad), or just to write documents (ever heard of the FreeWrite[1]?), or just to play games (I'd rather use a game console). Rather, I buy a computer because it's a single device that can do all of those things.
I almost added "and switch between them seamlessly" to the end of the last sentence, but of course on a Mac with Bootcamp, that's the one exception—you have to reboot in order to play a game. Still, on a laptop, I consider the ability quite valuable—I don't want to carry around a second computer just to play a quick game of Spelunky from time to time. If I couldn't do that at all on a Mac, it would be a point in favor of Windows.
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1: https://getfreewrite.com/