Why doesn't NVIDIA build a CPU? They'd have to license x86 from Intel/AMD (since most Windows apps are built for x64) but AFAIK CPUs are far less complex (in sheer amount of transistors) than GPUs.
CPUs need to make legacy code up to 40 years old run faster so there is a ton of complexity in the hardware. They are chasing people who want modest speed bumps without large changes.
Kinda like how apple was able to pull off a performant phone/tablet after Microsoft failed a bunch... Because they got people to rewrite apps (or create) for their platform instead of shoehorning windows apps into a different form factor. Much bloat was cut, usability was redone. It's an analogy so don't go silly over the differences.
If it were easy to build a CPU competitive with Intel, AMD would do it more consistently.
As an aside: I know it's not the done thing to complain about down votes, but I do find an honest, polite question - not even a statement - being modded to oblivion somewhat unusual.