MS bought the design rather than the chips so than it could avoid the headache of Intel refusing to lower the price of the chips, right?
It's a bit more than that.
Consoles rely on die shrinks for cost reduction, and ultimately combining pieces of silicon onto the same die.
If you buy from Intel, intel will produce the silicon in their own fabs, they usually don't just shrink existing designs for cost savings and how would you combine the CPU and GPU now or further down the line.
Consoles are as much or more about the cost 3-5 years from launch, as they are about the cost at launch.