The fact they are also C libraries is because that was the only real option of the time when they were written; rather like saying most car accidents were caused by black ones when the only option was a model T.
Before C escaped UNIX there were other systems programming languages available.
Turbo Pascal and Apple Pascal ruled back then, at least in Europe. On Apple's case it was even the main OS language.
However with UNIX being adopted at work and universities, many wanted to be able to transport their work between home and work/uni. So people started to slowly adopt C.
I followed the same pattern with C++. Eventually Turbo Pascal wasn't no longer an option, so started using Turbo C++, C was just too primitive and unsafe already back then, vs TP.
At least C++ had some of the safety features I was used from TP, as long as one stayed away from C style coding.
With Pascal, I recall having to use more assembler.
I had to use Assembly in both cases.