Maybe because Go devs are more allergic to 'non-Go' solutions that Java devs are to 'non-Java' solutions? (Explain: Java's xerial driver is a DLL/SO wrapped in a Java library)
Perhaps someone should define a new C compatible threading API to allow C libraries (including glibc or a wrapper around glibc) to work with something other than native pthreads. Such as goroutines or Java threads and so on.
Many general M:N threading solutions have been tried over the years. As far as I know current thinking is still that you need substantial cooperation from a language runtime to make it worthwhile. (And even then it's hard - Java's first attempt failed and they went 1:1 essentially between 1998-2022.)