Because right now you can only upload individual distributions to PyPI. A release is implicitly created when you upload the first one. If PyPI implemented that change, you could only have one distribution per release.

The proper fix would be to make publishing a release a separate operation. But that breaks all existing tooling and workflows.

Make it a configuration option for the package on pypi, then tooling can migrate slowly and at some point it can become the default for new packages. If then at some point someone uses old tooling the downside would be that they might need to do a manual publishing step to actually publish their package, but at least you don't have the problem of publishing before you are ready.