> why the standard library fs primitives don't use `at` functions under the hood

In this case it wouldn't seem to make sense to use `at` functions to back the standard file opening interface that Rust presents, because it requires different parameters, so a different API would need to be designed. Someone above mentioned that such an API is being considered for inclusion in libstd in this issue: https://github.com/rust-lang/rust/issues/120426