Might be a good reason.
Davin's and Steve's remarks below got me to thinking. I'll bet the motherboards, cases and some other components are proprietary, and wouldn't be sold at a price which would attract nosepickers such as me.
Might be a business opportunity - design a generic case, contract out for motherboards and displays, and start selling the stuff to make them.
My guess is that a lot of smaller OEM manufacturers would start creating compatible stuff.
Arf