Copyrights cover documentation, and may cover the names of instructions. But creating something that's interoperable with x86, and doesn't violate those copyrights, should be considered fair use. In the same line of copyrighted APIs vs. competing implementations of said APIs. Basically: make cpu that eats binary code of >20y old x86 cpu, use new names for instructions, and write your own documentation.
But of course recent, high performance x86 cpus are still covered by many patents.
So what's legal are competing implementations of x86 cpus released ~20y ago, not using performance enhancing technology covered by still-valid patents. To do what? No market for those.
Not to mention Intel & AMD have legal teams that could make the going tough for anyone trying to do so. Even if legally allowed.
A big advantage for RISC-V is being a clean slate ISA, meaning its design takes into account many issues that were bolted on ad-hoc in other ISAs (32/64 bit, extensions, virtualization, compact instructions, etc).
And being a modular design, you don't pay the power/area cost if you just need a simple core.
RISC-V is permissionless, you just grab the specs and start implementing (or more likely, grab a free core off Github and start modifying it). So that's the difference here.
For extensions, RISC-V supports those in a somewhat more structured way. There's both an open process for ratifying standard extensions, and a reserved space for your own custom extensions.