The problem is that each POSIX compatible system tends to have different levels of support for the various POSIX standard versions.

Additionally there are APIs which have undefined edge cases or different limits (e.g. amount of open file handles).

So even if you only target POSIX systems, you have to pollute the code with #ifdef to handle such differences.

It is similar to do web development. There are standards, but each browser version is a different world.