Why would Intel and AMD put a hole in the wall that guards their duopoly? Justifiably it wouldn't be a sales booster in the short term and in the long term it's a threat.
That was a side hustle. That was expanding their markets. They also tried with the Atom and the Xeon Phi, but this is now other architectures moving into their heartland. Sure, if RISC-V takes over then of course Intel will adapt, but right now they have the duopoly that they share with AMD. If RISC-V becomes a standard than IBM for example will get involved and they definitely know how to design a chip.
But this really looks like another industry shift like when businesses all moved to Unix, or when unixes all moved to business, or homes consolidated on Wintel, each time moving to a more open commodity platform. RISC-V and (to a lesser degree) ARM are much more commodity than the duopoly.