Most OS are written in C. Thus to interface to get good use from the OS you use the libraries which are in C.. (To get networking, shared memory, message passing, process scheduling and all that good stuff).

I wrote c wrappers for ada, and it was a pain. Much easier to use C (or C++) to get to OS functionality.

As long as OS are written in C, it will be with us. Also useful for embedded.

see: /usr/include