The thing is, Nvidia has "done it" and still no one else is "doing it". As the gp points out, Nvidia launched CUDA a while and this made Nvidia's products general purpose parallel processors.

They have succeeded as that and yet still no one is selling chips that either have an equivalent interface or (better) are drop-in Cuda compatible (since we know interfaces and programming language copyrightable).

The situation seems a bit like early PC days when companies sold (roughly) PC compatible computers were appropriately shoved aside by drop-in compatible clones/white-boxes. A company that makes a thing "kinda like" the leader but with it's own sauce is just trying to carve a piece of an existing market. The company that makes something exactly compatible with the leader has an incentive to go further. I hate to say "embrace and extend" but there you have it.