Comment by jcranmer

Libc is really a conflation of at least three different notional libraries. The first library is what its name suggests it is, a standard library for C. Another part of the library is in providing the userspace portion of system services--things like handling static initializers, dynamic loading, or the userspace side of things like creating new threads (not to mention, the actual raw functions you call to get the kernel to do something). The final part of the library is a collection of userspace services which are language agnostic, you might choose different implementations, but you'll always assume are somehow present--libm and malloc are the goto examples here.

As for why the userspace system service library is part of libc instead of being a separate libsyscall or libkernel, that probably is due to Unix being a C operating system--written at a time when most operating systems also came with their own system language. It's definitely not the case for all OS's that the C runtime library is the same as the libsyscall/libkernel--most notably, on Windows, the former is MSVCRT*.dll and the latter is kernel32.dll (or ntdll.dll if you're looking very specifically at syscalls themselves, but ntdll.dll is largely an unstable interface).