Hardware

And remember: all of AMD's R&D is between CPU, semi-custom, GPU, FPGA, memory control, etc. nVidia technically has a decent chunk of non-GPU-specific (tegra/SoC stuff like for the Switch, or high-speed networking interfaces) but still their GPU teams outnumber all of AMD's R&D by almost 10x alone.

Intel had a similar scale advantage but they for some reason stagnated for (depending on how you count) 5-10 years which allowed AMD to dig themselves out of their hole. nVidia at least hasn't been that incompetent, though they have stumbled quite a bit recently on their total inertia.