Because people like them. I've asked myself this question since a similar thread was posted a few months ago and I think I finally have an answer. Speaking, of course, only for myself: The G4 with Altivec is the first computer that did everything that I dreamt of as child that I would one day be able to do with a computer. It's hard to break the emotional bond that is built under such circumstances. When something comes along that fulfills your wildest dreams, you tend to think very highly of that particular thing. I do have newer machines, both Linux and Mac, but PowerPC continues to meet nearly all of my needs and is still the computer that I spend the most time on.
EXACTLY! I have two Macs that are newer and faster than the Powermac G5 Quad that I now use daily, but there is something about my G5 Quad... I remember how blown away I was by Mac when I got my first G5 - simply amazing compared to Windows. That emotional attachment continues to this day. I started out on G5 and Tiger, and that is where I have returned to. Some day I will be forced to augment, as some software need comes up that can't be met on a G5, but until then, I am "home".
For fun, I am restoring an old Power Macintosh 7500, which I have upgraded to a G3/400 MHz. The result is an amazingly useful machine, although I have to note that Mac OS 8.6 does not exhibit the same rock solid stability that Mac OS X does. Still, it is fun and useful, and has great vintage games, such as Barney Blaster!