Yeah, but you’re missing the specialization angle.
NVIDIA and Intel as companies are specialized in the design (and in the latter case, manufacturing) of chips. Board OEMs are specialized in making a consumer-ready product, maintaining worldwide sales and distribution channels, and consumer relations.
Of course, it wouldn’t be impossible for NVIDIA to start doing these things on their own (see Apple, who designs chips, designs computers around those chips, and operates retail stores where those computers are sold), but presumably NVIDIA prefers the current arrangement, where they can just focus on the chips and leave the rest to OEMs.
See also Intel under Gelsinger, who sold off the NUC and server lines (finished products) to focus on the core business (x86 chips).
Ironically, Intel's GPU business seems to be entirely in-house. Though maybe it too will get spun off in whole or in part.
As far as nVidia is concerned, they lost the privilege to be treated like a small fabless startup. They are regularly ranked as the highest valued company on the U.S. stock market. They clearly can make and sell the whole card themselves, so having GIGABYTE, ASUS, and co. hang around and take the heat for their business decisions feels pretty scummy. It's also clearly bad for the consumer, as Founders Edition cards actually do sell for MSRP. This partner crap is all an obsolete relic of a bygone era, being drawn out well past its prime.