Because they are annoying and unnecessary additional work. If I write something, I won't know the dependencies in the beginning. And if it's a personal tool/script or even a throwaway one-shoot, then why bother with managing unnecessary parts? I just manage my personal stack of dependencies for my own tools in a giant env, and pull imports from it or not, depending on the moment. This allows me to move fast. Of course it is a liability, but not one which usually bites me. Every some years, some dependency goes wrong, and I either fix it or remove it, but at the end the benefit I save in time far outweighs the time I would lose from micromanaging small separate envs.

Managing dependencies is for production and important things. Big messy envs is good enough for everything else. I have hundred of script and tools, micromanaging them on that level has no benefit. And it seems uv now offers some options for making small envs effortless without costing much time, so it's a net benefit in that area, but it's not something world shattering which will turn my world upside down.