It's much more advantageous to be able to respin/redesign parts of the GPU for a new architecture since the user interface is at a much much higher level compared to a CPU. They basically only have to certify that it'll be API compatible at CUDA/OpenCL/Vulkan/OpenGL/DirectX level and no more. All of those APIs specify that the drivers are responsible for turning it into the hardware language, so every program is already re-compiled for any new hardware. This does lead to tiny rendering differences in the end (it shouldn't but it frequently does, due to bug fixes and rounding changes). So because they aren't required to keep that architectural similarity anymore, they're free to change as they need new features or come up with better designs (frequently to allow more SIMD/MIMD style stuff, and greater memory bandwidth utilization). I doubt they really change all that much between two generations, but they change enough that exact compatibility isn't really worth working at.

If you want to look at some historical examples where this wasn't quite the case, look at the old 3DFX VooDoo series. They did add features but they kept compatibility to the point where even up to a VooDoo 5 would work with software that only supported the VooDoo 1. (n.b. This is based on my memory of the era, i could be wrong). They had other business problems, but it meant that adding completely new features and changes in Glide (their API) was more difficult.