Because there isn't any other operating system that makes all the things I want to do with a computer any simpler. We've got only 3 major desktop operating systems, and they're all overly complex and poorly designed in lots of ways.

Since the late 1990's, I've alternated: I use a Mac for a while until I get fed up with it, and then I use Linux for a while until I get sick of that, and then I go back. (I've used Windows at work, and it's not any better.) I just need enough time to forget how much I hated the other one.