I'm not an expert and might be horribly wrong and welcome edification, but I believe you don't want your GPU powering your computer because - GPUs don't handle

I'm not an expert and might be horribly wrong and welcome edification, but I believe you don't want your GPU powering your computer because - GPUs don't handle I/O other than writing to video RAM. - GPUs don't handle interrupts. - GPUs don't handle branching well. For the first two, a lot of infrastructure that is part of the chipset and platform would have to be extended to each compute unit of the GPU. Imagine a database on a GPU. It's not like 1.5K-2k+ cores that are super good at math can read and write your disk or disk array at once.