18 core Xeon chips have been available for 3+ years. There is nothing particularly odd about Apple's choice -- AMD has not been a serious competitor in the workstation market for years, and even with their newest entrant are only nipping at the very bottom of the market.

Also, no Thunderbolt from AMD.

Thunderbolt.

Yeah, probably the biggest reason. Maybe this will change when Thunderbolt will be royalty-free next year. AMD has the advantage of being the only company on this planet which can combine a high performance x86 CPU and high performance GPU.

I'm kinda hoping for Thunderbolt PCIe cards that aren't tied to a specific brand or board. Would be nice to just throw an x4 or x8 card into an existing system to get one or two TB3 ports for stuff like external storage.

I believe that OSX is optimized for Intel's brand of x86 - might have been too much effort to switch vendors when it comes to future kernel support.

macOS does not seem like an OS that has been optimized much at all in regards to the CPU.

Also, Apple dragged macOS from PPC to x86; switching x86 implementations is no big deal for the rest of the world, it surely should not be a challenge for Apple.

Yeah. People have done AMD Hackintoshes. It's more likely that they have some agreement with Intel that prevents them from going AMD for the CPU.