That's because many people don't pay attention to reproducibility of their developed software. If there is no lock file in a repo that nails the exact versions and checksums, then I already know it's likely gonna be a pain. That's shoddy work of course, but that doesn't stop people from not paying attention to reproducibility.

One could argue, that this is one difference between npm and such, and what many people use in the Python ecosystem. npm and cargo and so on are automatically creating lock files. Even people, who don't understand why that is important, might commit them to their repositories, while in the Python ecosystem people who don't understand it, think that committing a requirements.txt only (without checksums) is OK.

However, it is wrong, to claim, that in the Python ecosystem we didn't have the tools to do it right. We did have them, and that well before uv. It took a more care though, which is apparently too much for many people already.