Wouldn't C++ and Rust eventually call down into those same libc functions?

I guess for your example, qsort() it is optional, and you can chose another implementation of that. Though I tend to find that both standard libraries tend to just delegate those lowest level calls to the posix API.

Rust doesn't call into libc for sort, it has its own implementation in the standard library.

Many of the libc functions are bad apis with traditionally bad implementations.