Because polluting the global namespace is considered a bad thing. You never know if your naming conventions don’t overwrite something else or vice versa.
That’s why basically anything with the capability of importing external modules has namespaces and local scopes - that way you can do anything you want in your file/module and don’t have to worry about breaking things in another file/module (besides the usual ‘Called a method in the wrong way’, of course).
And, yes, it’s slightly annoying to have to reimport stuff in every file. Then again, the alternative is potentially breaking things so there’s that.
In any way, this is a convention designed to “protect” you from yourself (or others). Not in the sense of security but in the sense of more sensible code.
It also makes it possible to easily see where code comes from. “Magic” variables/objects like the one you’d like to create makes it harder for you (or others) to see what’s going on if you revisit your code at a later date.
tl;dr: Don’t. It’s a bad habit and should only be considered if there’s absolutely no other way.