Because that happened in a time when there were many players in the 90s each making their own GPUs for gaming purposes specifically. Compute was not even on the picture until things like CUDA and OpenCL came out.
Sony copied that idea for the 1st Playstation, and then folks like NVidia & 3DLabs quickly followed suit, the idea being they would enable that functionality for games like Final Fantasy.
In the early 2000s, the HPC folks realized that you could use a GPU for physics & engineering codes, and here were are 20 yrs later.