This isn't true, libc is magnitudes better defined than binary names in shell scripting, but it's still yet another case of approximation-API with multiple competing implementations.
There are plenty of ecosystems where programs declare a specific library implementation they expect to call into (Rust, Python, Npm, Ruby, Perl, ...) often even constrained by versions. But also if you depend on libcurl you are only going to have to deal with multiple verions of the same implementation (that you can still constrain in e.g. pkg-config).
In shell scripting you have to deal with stuff like "in nc, EOF on stdin is undefined behavior and implementation specific".
I don't see the difference? There's a POSIX spec for coreutils (and some other shell-usable executables), there's a POSIX spec for libc (and some other C libs), and both are a somewhat painful subset of what most programs/scripts actually use. And yes, in both cases often the solution is to explicitly depend on a particular implementation or even version thereof; systemd explicitly only supports glibc, shell scripts targeting macOS may require that you install a newer version of bash than the OS ships, and yes, if you need nc to behave a particular way then you need to depend on the right one (I've actually personally hit that; IIRC I installed netcat-openbsd but it didn't include a feature I needed). In all cases, there may be a portable standard, but it doesn't cover everything, so if you're not inside what's covered by the standard then you should specify your actual dependencies. It still doesn't matter whether the API you're touching is mediated by the link phase of a compiler chain or *sh calling particular binaries.
There are plenty of ecosystems where programs declare a specific library implementation they expect to call into (Rust, Python, Npm, Ruby, Perl, ...) often even constrained by versions. But also if you depend on libcurl you are only going to have to deal with multiple verions of the same implementation (that you can still constrain in e.g. pkg-config).
In shell scripting you have to deal with stuff like "in nc, EOF on stdin is undefined behavior and implementation specific".