The Xbox doesn't exist to support DirectX, most AAA games write to bare metal for better performance.It exists because Microsoft wanted a living room presence the way the Soviets coveted a warm water port.
There is no bare metal on Xbox, you always go through DirectX. The difference is that the Xbox version of DirectX has much lower overhead than the Windows version and provides access to all the functionality of the GPU, not just whatever's in the comparable PC version of DirectX.
