True, you could, but I would hazard a guess that most people don't want that hassle (and it is a hassle, though using Linux is far easier these days than it used to be. It's still not something I would suggest to my mom).
User interface preference aside (and I agree they look a lot alike these days), there are real structural OS differences between Windows and OS X. The difference between an OS that uses a registry and one that doesn't is pretty significant, for example. Whether or not that matters to the average user - probably not. But there are more differences than just the GUI.
I think a lot of it boils down to personal preference. I've been using Windows since, well, before it was Windows. I've used every version except Vista and Millenium. I thought XP was actually very good. It was certainly far more stable than any previous version of Windows. Vista, on the other hand, was a disaster. That's where Windows lost me. Windows 7 offers some redemption, but at this point it's not enough for me. My tolerance threshold for certain Windows aggravations is a lot less than what it used to be. Why, for example, on a clean install of Windows 7 would it take me so much effort to get it to sync with a Windows Mobile phone (stock ROM, no funny stuff, 1 year old phone)? Wouldn't you think two stock Microsoft products would play nicely together? It took forever to get it to work. Finally after trying multiple attempts, with multiple reboots of the computer and the phone and reinstalls of the synching software, it "just worked." Ha.
But I think the whole Mac v. PC debate usually ends up being rather pointless. Both have pros and cons. I think in a lot of cases people haven't really used both platforms extensively enough to really compare and are very caught up in the idea of adamantly disliking just the idea of the other one. For me, I'm pretty firmly in the Mac camp these days, but I'll keep an eye on what's going on on the other side. I've switched once and I'm sure I could switch again, but I would have to have a really compelling reason to do so.