One word: antitrust. The discrete GPU market these days consists of Nvidia and AMD, with Intel only just now dipping its toes into the market (I don't think there's anything saleable to retail customers yet). Nvidia buying AMD would make it a true monopoly in that market, and there's no way that would pass antitrust regulators. Nvidia recently tried to buy ARM, and even that transaction was enough for antitrust regulators to say no.
There really used to be a lot of intra-generational tweaking and refinement, like if you look back at Maxwell there were really at least 3 and I suspect 4 total steppings of the maxwell architecture (GM107, GM204/GM200, and GM206 - and I suspect GM200 was a separate "stepping" too due to how much higher it clocks than GM204 - which is the opposite of what you'd expect from a big chip). Kepler had at least 4 major versions (GK1xx, GK110B, GK2xx, GK210), Fermi had at least 2 (although that's where I'm no longer super familiar with the exact details).
Anyway point is there used to be a lot more intra-generational refinement, and I think that has largely stopped, it's just thrown over the wall and done. And I think the reason for that is that if NVIDIA really cranked full-steam ahead they'd be getting far enough ahead of AMD to potentially start raising antitrust concerns. We are now in the era of "metered performance release", just enough to stay ahead of AMD but not enough to actually raise problems and get attention from antitrust regulators.
Same thing for the choice of Samsung 8nm for Ampere and TSMC 12nm for Turing, while AMD was on TSMC 7nm for both of those. Sure, volume was a large part of that decision, but they're already matching AMD with a 1-node deficit (Samsung 8nm is a 10+, and the gap between 10 and TSMC 7 is huge to begin with) and they were matching with a 1.5 node deficit during the Turing generation (12FFN is a TSMC 16+ node - that is almost 2 full nodes to TSMC 7nm). They cannot just make arbitrarily fast processors that dump on AMD, or regulators will get mad, so in that case they might as well optimize for cost and volume instead. If they had done a TSMC 7nm against RDNA1 they probably would be starting to get in that danger zone - I'm sure they were watching it carefully during the Maxwell era too.
(the people who imagined some giant falling-out between TSMC are pretty funny in hindsight. (A) NVIDIA still had parts at TSMC anyway, and (B) TSMC obviously couldn't have provided the same volume as Samsung did, certainly not at the same price, and volume ended up being a godsend during the pandemic shortages and mining. Yeah, shortages sucked, but they could still have been worse if NVIDIA was on TSMC and shipping half or 2/3rds of their current volume.)
Of course now we may see that dynamic flip with AMD moving to MCM products earlier, or maybe that won't be for another year or so yet rumors are suggesting monolithic midrange chips will be AMD's first product. Or perhaps "monolithic", being technically MCM but with cache dies/IO dies rather than multiple compute dies. But with RDNA3 AMD is potentially poised to push NVIDIA a little bit, rather than just the controlled opposition we've seen for the past few generations, hence NVIDIA reportedly moving to TSMC N5P and going quite large with a monolithic chip to compete.
Nvidia actually used to develop chipsets for AMD processors include onboard GPUs, they did for Intel as well but they had a much more serious relationship with AMD in my estimation. This stopped with the ATI purchase since ATI is nvidia's main competitor the two companies stopped working together. Intel later killed all 3rd party chipset altogether and AMD had to do a lot of chipset work they weren't doing before.
I sometimes wonder what would have happened if they had merged back then. I personally think a Jensen Huang run AMD would have done much better than AMD+ATI did in that era. I could easily see ATI having collapsed. What would the consoles use now? Would nvidia have been as aggressive as it has been without the strategic weakness of now controlling the platform it's products run on?
Sure, the new owners could re-negotiate with Intel, and maybe nothing would change. But who knows? A combined AMD/nVidia might be a sufficient threat to Intel they might pull some desperate moves.
(In some timeline, this turns out to be the boost that makes RISC-V the new "standard" ISA, but I am not so optimistic it is the one we live in.)