Not enough demand. No evidence that Apple would sell enough of them. We're not talking about tech enthusiasts, gamers (apart from the vast console market), or Pros here. We're talking about the average user, families, etc. That is, the biggest chunk of the market. People for whom a "video card" is something you hand over at Blockbuster to rent movies with. And really, why should it even be any more complicated than that for them? Besides, there is no point at all in playing around with new desktop lines when the market is headed in the direction of mobile power. Consumer desktops as we understand them now are heading into obsolescence.
a lot of assumptions on your part. I don't think its lack of demand, because folks can just go look at all the other windows based options that allow for a greater hardware selection that is upgradable. I don't think people want to throw down for a Mac pro when all they probably want to do is be able to add a couple more devices, swap out drives possibly, RAM, HD, Firewire ports etc. to assume the general public is too stupid because apple doesn't deliver is a bad idea. Just because apple doesn't offer it, doesn't mean people don't want it.