You are still confusing the OS Feature with the language and not considering the language type.
C/C++/FORTRAN.... Are Compiled. There is a program (like GCC) that will interpret the text and convert it into Machine code
These libraries are often compiled in C just because it is a popular language for compiled code. However it doesn't have to be. There are other IPC processes in the OS that we can choose to communicate with other programs so you C code can talk to a Python Program or include your favorite java library as well. As well most of the languages will allow you to go the other way as well.
However it is rather poor form to mix languages (I am of the computer science discipline), I even shy away from including standard linux libraries in a python app, because it makes your code base much more difficult to maintain having to deal with multiple compilers and complex builds and difficult to trace problems, as well harder to distribute.
But to clarify my original point about reusability it was more about the programming discipline of trying to keep your functions and methods more generic and flexible so you don't have a lot of similar functions with minor tweaks. And this isn't a rib against the language it is just the Computer Engineering programming methodology will put value in generic code but not at the expense of performance. While the Computer Science methodology will be more willing to tradeoff performance for less rewriting code in the future. These are tradeoffs that we have to choose to make, they are not necessarily better or worse but people who follow a particular discipline have formed their habits and will default to what they know. Sure with my Computer Science Discipline will have to break the rule and make a particular function for only one job because it needs to run fast because I see it is a bottleneck. And yes the Computer Engineers will make generic functions as they know it will be needed over and over again other wise they will be spending too much time coding the same stuff.