The latter being a market Apple has never wanted to get involved with. Gamers are notoriously picky about hardware and are inherently incompatible with Apple's approach of producing a solid and simple line of products. No configuration Apple could offer would match the flexibility they could get by building their own systems to suit their tastes.Because the Mac Pro is a workstation not a desktop. The Mac Pro is aimed at professional while this new desktop would be aimed at the home user/gamer. Two totally different markets.
You'll note that Apple has deliberately and carefully produced products that are anything BUT the vanilla desktop or the plain production-line notebook. They don't want to court the bottom-dollar people or the builders who demand extensive choice, and they shy away from making products that could be seen to compete in those segments.
That's why the PowerBook went for thin and light (A 1.1" notebook in 2003?). It's dramatic and aesthetic but the PowerBook itself was rarely anything more than middling specs and performance. That's why the Mac mini is so small. It's not just a budget PC--it's a silent, stylish complement to a room. That's why the iMac isn't just a mid-range tower with a monitor.
For all of these things, just looking at the computer from the standpoint of component specs neglects everything that makes an Apple computer an Apple computer. Gamers care about specs (they care about revisions within single components, even), and for good reason, and so they should continue building their own systems or getting standard towers and swapping out parts. They're not meant to buy a Mac for their killer game system. You know, the right tool for the right job and all that.
It's not Apple's way, and I wish people would stop trying to shoehorn it in. Computers are built into the design at Apple, rather than designing computers around the components. You can't have a wildly expandable, flexible system with that approach--Apple can't make other hardware magically fit their constantly evolving designs.