Because vim (and emacs too!) is a powerful editor that works purely on command line and most people who are experienced in the Unix world are generally familiar with it. There are plenty of easier editors to use, like nano, that are also widely distributed and you are free to change your default. Being really powerful but you kind of need to know what you're doing is basically unix's whole thing.

Nano is the default on Debian for more than a decade. Maybe two. I don't think vim is the default on any largely use distro now.

Are you actually asking why people use them?

I guess some people really like the features. Imo Micro/Nano are much easier to use than Vim

Why are these hard to understand editors still the default on most distros and flavors

I think nano is usually the default nowdays. Nano os pretty minimal and has it's keybinds always on display so you don't need to memorize them.

Why haven't they reinvented themselves with easier to understand shortcuts?

Nothing about vim and alternatives feels intuitive or easy to use

(Neo)vim doesn't need to reinvent itself to be more accessible, because it does what it does very well. I'm a web dev and have used vscode like anybody else for a long time. I decided to try neovim because vscode was performing badly, but kept me using it because of how good the developer experience is. Once you learned how to use it, there is just nothing better.

but when every other software with keyboard shortcuts agrees on certain easy to remember standards, I don't quite understand how software that goes against all of that hasn't been replaced or hasn't reinvented itself in newer versions

In a way, it has been replaced. Most people will use a user friendly IDE and ignore vim. The thing about vim is that it does things in a fundamentally different way than any other editor, so reinventing itself would mean loosing everything that makes it good, then you better off using something else.

Then again, I have no idea what the difference between vi, vim, emacs, and nano are

Nano is a simple, easy terminal text editor; vi, vim and neovim are three versions of the same quirky and hard, but very good text editor/IDE; emacs is a quirky, but kinda bad editor that has amazingly good extendability.

I'm not the most Linux savvy but when I ssh onto a work machine I'll use a terminal editor instead of copying the file onto a local machine, editing the file in a GUI and then overwriting the file on the remote machine