I remember being in uni in the early 2000's and going to a conference where one of the presentations was how they tricked a GPU into "rendering" an image that was basically a simulation of fire propagation, and how it was faster (about an order of magnitude) than the fastest CPU simulator. They had a few issues because it was complicated, and the texture data type were not exactly what they needed, and, and, and...

And then CUDA emerged.

So it's not like they magically thought of GPGPU -- there were people working on it before CUDA, but yeah, they had the vision and invested the money and hours of work to make it happen.

I wrote a shader to do computation once. It wasn't too bad actually.