Why does file fragmentation not occur on Apple Macintosh computers like with Windows computers?

I was just wondering, because I've never heard of having to defrag my Apple Macintosh files. I've had my Macbook Pro for a year and a half, and I never have to do any maintenance of that kind. I was wondering why people would buy a PC when they could have this kind of stability.
Update: Oh, and please mark this question as interesting if you own a Mac. We Mac users have to stick together in this war against Big Brother.
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