There's not that many. C/C++ and Rust all map to OS threads and don't have CSP type concurrency built in.
In Go's category, there's Java, Haskell, OCaml, Julia, Nim, Crystal, Pony...
Dynamic languages are more likely to have green threads but aren't Go replacements.
If you want mainstream, Java and C# are mainstream and both are used much more than Go. Clojure isn't too niche, though not as popular as Go, and supports concurrency out of the box at least as well as Go. Ada is still used widely within its niches and has better concurrency than Go baked in since 1983. And then, yes, Erlang and Elixir to add to that list.
That's 6 languages, a non-exhaustive list of them, that are either properly mainstream and more popular than Go or at least well-known and easy to obtain and get started with. All of which have concurrency baked in and well-supported (unlike, say, C).
EDIT: And one more thing, all but Elixir are older than Go, though Clojure only slightly. So prior art was there to learn from.
Erlang (or Elixir) are absolutely Go replacements for the types of software where CSP is likely important.
Source: spent the last few weeks at work replacing a Go program with an Elixir one instead.
I'd use Go again (without question) but it is not a panacea. It should be the default choice for CLI utilities and many servers, but the notion that it is the only usable language with something approximating CSP is idiotic.
> There's not that many.
You list three that don't, and then you go on to list seven languages that do.
Yes, not many languages support concurrency like Go does...