Hmm. I'm speculating, but I'd guess that tossing on the same external controller that they've put on a million other motherboards in the past probably actually saves cost, vs. designing something unique to expose the on-die controller.

I wonder what the performance impact is, though. I'd guess (very naively) that an on-die controller would perform better.

My guess is that the Intel Ethernet drivers are seen by the customers as very tried-and-true, and how to tune them is well-documented. AMD's Ethernet drivers are relatively unknown and unproven, and it's easy to imagine that they could perform worse. That could easily make up the difference in cost between an external PHY vs an external MAC+PHY.