Arguably the other reason for a lack of a default upper bound stems from PyPI has never required semver for hosted packages, there are plenty of packages on PyPI still using calver or marver or other more bespoke version schemes. (Whereas npm has always made semver an assumption/"requirement" for hosted packages.)

As someone part of the problem with a couple ancient packages in PyPI that are calver (though arguably I'd be surprised if many depended on them), I wonder if it is too late for more of the Python ecosystem to shift more directly to semver by default/everywhere as some of the other ecosystems now are.