I think you missunderstand something...
You can get an Intel mainboard that supports both, with SLI or Crossfire. You can also get an AMD mainboard that does the same.

AMD&ATI are the same company, and this gives them a big head-start in the all-in-one chip market that is just around the corner (both AMD and intel now designing multi-core chips with mid-high end graphics built in). Intel knows that they cannot compete with ATI in graphics (Intel's latest and greatest video is not as good as a 3-year-old ATI laptop video chip). Their only solution was to band together with Nvidia. Nvidia also benifits from this, as they will recieve a lot of technologies from intel's CPU devision (ATI took off a few months after AMD bought them for the same reason).