We did back in 2007: https://en.wikipedia.org/wiki/Open_Graphics_Project

And there have been some others as well: https://en.wikipedia.org/wiki/Free_and_open-source_graphics_...

Recently https://www.furygpu.com/

Part of the problem is that every ASIC manufacturer (and indeed each fabrication process) has a different toolchain with a different set of primitives for circuit design. Yosys and other open tooling for FPGAs has helped a great deal in lowering the barrier to chip design and by association reuse of circuits. But every ASIC, at the moment, is tied to some vendor's PDK. Here's the one Google open sourced for Cypress Semi's SKY130 process node: https://github.com/google/skywater-pdk

It is at least theoretically possible to build a headless "GPU" from RISC-V processors that have the vector extension (RVV). RVV had been designed to be able to run programs compiled for the SIMT execution model that most GPUs use.

This Orange Pi RV2 has a small vector unit in each core, and could be used for at least prototyping the software until more powerful chips are available.

BTW. There have also been a couple hardware startups that have been working on commercial GPUs based on RISC-V's vector extension, with their own GPU-specific instruction set extensions for texture lookup and the like.

It's probably a series of patent landmines...