Because uploads to PyPI are not atomic. They’re now capped within a 14 day window, but it would be extremely confusing to users to have the “release” hash of their dependencies change repeatedly.

(Also: we’d need to determine what it even means to hash a set of files. Do we order by canonicalized filename, by upload time, etc. Each of these has surprising implications!)

Edit: to be clear, it’s not ideal or fully intentional that uploads to PyPI are not atomic. But it’s the status quo and any change will take years.

The semantics are important. A release is composed of multiple distributions (sets of files). Each distribution does have a one-way hash value. This is what you lock to. Adding files does change the value of that hash. Files cannot be removed or changed.

You lock to the distribution instead of the release so your build doesn't have to download distributions for platforms you're not using.