Because I'd never go back to using a commercial operating system, and gentoo is the best free solution... It's better than the non-free ones aswell
I like having complete control over my system, even though I'm no developer and I'm not getting the absolute best performance from it. And ever since I started using gentoo exclusively I haven't had to upgrade my system, it's been about a year-and-a-half. Portage is probably the best part. It's so easy to find and install packages. It's like a big box full of software and it's all free. (Except for those commercial games

)
I first tried red hat 8 a real long time ago, it was my first linux attempt. I had no clue what was going on. I think I installed gnome
and kde

Anyways, I had it installed for about a week and then got rid of it and went back to windows. red hat seemed like windows but without as much software and hardware support. Then, a year or 2 later I got a 64-bit processor, the same one I have now, an Opteron 146. I wanted to try 64-bit so I downloaded the suse dvd

Took really long. Then I installed it and thought it was pretty neat, it had a lot of cool stuff on it, so I definitely thought it was better than windows, but it's owned by novell, and I really don't want to use their software (person reasons

). Plus I found it quite slow, althought the YaST control center is cool.
So then someone recommended gentoo and I then tried 3 times to install it. Once I got it working I stuck with it for about a month and realised I had a really bad install. So I tried again with all the stuff I learned from my first attempt, and that's my system now. I learned so much about linux from gentoo and I can apply all that to my system. Not that I'm much a performance guy. I basically just use my computer for music, instant messaging, email, internet, and sometimes video games.
Oh, and I love being able to tell my high school teachers that KOffice can't create .doc files.