The "libc" is more than libc.so. Your system headers, for example, must be shipped with your libc, even though everything that would change if you replace them

The "libc" is more than libc.so. Your system headers, for example, must be shipped with your libc, even though everything that would change if you replace them with something nonsensical would be in "your" binary, not in libc.so. libc functions don't stop coming from the libc if you statically link them. A C runtime is often provided by the same project as the C standard library (the other viable option is to have it provided by the compiler project).