If you haven't noticed, there are plenty of standards kicking around in graphics and there isn't a need for any more. AMD, Nvidia, and Intel do a pretty good job supporting all of them - you can write a DX12 (or opengl, or Vulkan) program and run it any any vendor's hardware.

What problem do you think having a standard ISA would solve? Nobody distributes native binaries for GPUs, only shaders (either compiled to intermediate representation or no). From my perspective, all that a standard ISA would cause is less implementation flexibility in the hardware since now the hardware has to deal with compatibility instead of letting the vendor-specific compiler included with the driver deal with it.