There are plenty of them, the FOSS culture just tends to ignore them.

Microcontrollers usually are not POSIX-compliant. Android and iOS are not POSIX-complaint.

The ANSI/ISO C standard library and the POSIX standard are two different things though that just happen to overlap here and there.

My point is that a lot of people treat the POSIX standard as if it was the C standard library. It's understandable because of how limited the actual standard library is, but it leads to people having a view that an OS is doing things wrong if it's not following POSIX.

I think the proliferation of POSIX-like OSes is mostly down to OS projects wanting there to be useful software that can run on their OS, so they minimize/eliminate the work of porting. It has led to the phenomenon you describe though -- instead of thinking an OS is just too much work to port to, it might be thought of as being "wrong".

Worse is when the OS does follow POSIX but people haven't read the fine print that certain POSIX APIs are implementation dependent, e.g. signal handling and threads.