Also because there's a mismatch between goroutines and C threads as described here https://www.cockroachlabs.com/blog/the-cost-and-complexity-o... while Java threads can map 1:1 to C threads.

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.)