Hardware

Security is an interesting reason that most people don't think about.

When you run a program on your computer, you're constantly swapping between user and privileged modes. For example, you don't want a website reading the stuff from your harddrive. Any such attempts must go to the OS which will then say the website doesn't have permission and refuse.

GPUs don't have a privileged mode. This isn't just because it wouldn't be useful. To the contrary, webGL or WebGPU have massive security issues because you are executing third-party code using drivers which themselves generally have root access. GPUs don't add such a mode because their hardware takes forever (in CPU terms) to get all the data loaded up and ready to execute. Moving to a different context every few microseconds would result in the GPU spending most of its time just waiting around for stuff to load up.

The solution to this problem would be decreasing latency, but all the stuff that does that for you is the same stuff that makes CPU cores hundreds of times larger than GPU cores. At that point, your GPU would just turn into an inferior CPU and an inferior GPU too.