The key is form factor...PC’s operate from small net books to large monster gaming towers. There are small home theater PC’s nearly silent in operation and the size of a shoe box. There is a whole industry that supplies parts, cases, hard drives ect so that folks can build their own PC’s according to need and best price. Windows 7 can operate in all of them. Apple computers don’t have nearly the variety of form factors that can fit in a majority of user experiences. There certainly is no way to legally build one’s own customized Apple machines as the parts industry is almost exclusively designed for PC’s.
I think most folks see mobile products as adjuncts to their main systems, like smaller craft attached to mother ships that go out and come back again to be synched and upgraded when necessary. Apple’s mobile systems still have to play nice with MS systems or else they would again risk being marginalized by other competing mobile platforms.
Try squeezing dual 21” monitors into an iPod.