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.