Simple!
Size + Weight + Battery Life = Portability
But desktop PC components are getting smaller and smaller - soon you'll be building your own laptops! Hah, nah, that wouldn't work too well. But cost-wise, it's not worth it. Haven't you noticed? The graphics cards are getting bigger...hmm, if you go external perhaps, but even then, it should have a PCI-E connection to the board. Maybe your battery will last 5 minutes!
...the above makes no sense, but yeah.