First, x86-64 also has “extensions” such as avx, avx2, and avx512. Not all “x86-64” CPUs support the same ones. And you get things like svm on AMD and avx on Intel. Remember 3DNow?

X86-64 also has “profiles” which tell you what extensions should be available. There is x86-64v1 and x86-64v4 with v2 and v3 in the middle.

RVA23 offers a very similar feature-set to x86-64v4.

You do not end up with a mess of extensions. You get RVA23. Yes, RVA23 represents a set of mandatory extensions. The important thing is that two RVA23 compliant chips will implement the same ones.

But the most important point is that you cannot “just use x86-64”. Only Intel and AMD can do that. Anybody can build a RISC-V chip. You do not need permission.

Because the ISA is not encumbered the way other ISAs are legally, and there are use cases where the minimal profile is fine for the sake of embedded whatever vs the cost to implement the extensions

> why not just use x86-64?

Uh, because you can't? It's not open in any meaningful sense.