The problem with `setenv` is that people expect one process to have one set of environment variables, which is shared across multiple languages running in that process. This implies every language must let its environment variables be managed by a central language-independent library -- and on POSIX systems, that's libc. So if libc refuses to provide thread-safety, that impacts not just C, but all possible languages (except for those that cannot call into C-libraries; as those don't need to bother synchronizing the environment with libc).

In some cases this is true. In the case of setting and getting env vars, it is not. There is no comceivable reason for making a process that spends any significant portion of its runtime calling setenv() or getenv(). Even if those calls were a thousand times slower than today, it would still be a non-issue.