How did it come to be that only two companies supply all of the world's PC graphics chips?

I know Intel is dipping its toe into the GPU market, but let's be real, AMD and nVidia are the only options and have been for the last 20+ years. The manufacturers/assemblers of the complete graphics cards are varied and widespread, but the core tech comes from two companies only.

Why is this the case? Or am I mistaken and am just brainwashed by marketing, and there are in fact other viable options for GPUs?

Cheers!