Why? You assume that they would sell them at a cost that would enable you to build a Mac for less than Apple sell them. Lets look at the costs:
Assume Apple were willing to sell you the Motherboard with a bundled copy of OSX for $400 (yes this is a massive premium, but Apple need to protect their margins).
You now need to get an Intel Core CPUm, say $200.
RAM will set you back $100 at least.
You need a graphics card. I don't know which cards Apples Intel drivers support, lets assume you need a Mac specific card (it'll need to be EFI not BIOS supporting in it's ROM), say $200.
So we are at $900, you still need a case, optical drive and harddrive, say you can get all three for $150.
So $1050 and no monitor. Or just buy an iMac.