Same reason I still use my C64 (which is on the same desk with my Athlon 1800, A3000, and A500)... Pure nostalgia. There's nothing like booting and playing a game of Lemmings or Leander on the original format!
I've just recently returned to the Amiga world after finding a very cheap 3000 system (my dream system from back in the day). It's just been fun to play with it and re-learn how things work and see the current state of things. It's almost like the Amiga still has a soul while the pc world is just all business and no fun. I've built countless Athlon and Intel systems with all the latest toys, but just putting a Picasso II and Cybergraphx in my 3000 was the most fun I've had doing installs in a long time!
Bottom line, it's just fun!
98PaceCar