A very good question!
Because its different perhaps, because of nostalgia, and I'd hazard for many users the reason would also be that certain applications are not available on any other platform.
I must confess tho', when I look through what people are selling on ebay or through Computer Shopper etc, I am amazed at how cheap it is to build a PC.
As I pointed out the other day, someone paid 330 UKP/ 500 USD for a Cyberstorm PPC card - at these kinds of level of cost, it does make you wonder why they just don't buy a PC...