It's not all that surprising. Both AMD and Nvidia cater to DirectX runtimes. I look at DXR as Nvidia's latest gotcha for disruptive tech. Last few examples from AMD were FreeSync and before that Eyefinity. Remember when Eyefinity came out it took 2-3 years before Nvidia specifications cough up with Nvidia Surround. Won't be surprised if AMD has to take 2-3 years in order to properly bake in DXR into the chipsets. Chipset designs take years. What I find a bit disturbing is that we didn't hear about DXR from Microsoft until after RTX was a buzzword. That tells me that Microsoft is playing games with neutrality. That's not good for consumers.

Seeing as AMD seems to be focusing on mid-range video cards as their primary and that functional RT at decent frame rates is really restricted to the realm of top tier cards, it wouldn't make much sense to incorporate it into their lineup.  If it catches on and the tech matures, the next gen or two might make it more feasible.