The problems are:

requirements.txt is way too flexible, so users want a lock file to freeze stuff in place.

You can use pip freeze to generate a sort-of-lockfile but it's more manual than tools like Cargo/npm/bundler which do it automatically. And more adhoc as to what you call it. Combined with needing to mange venvs and sourcing etc. people want a standard script that just figures out its context from the now standard "explicit requirements"/lockfile setup.

It doesn't handle versioning Python itself, which is expected of it for some reason, even though nobody cares that npm/Cargo/bundler don't version their languages.