Not only does the editor need to track project folders and open files, even something seemingly simple like syntax highlighting can be surprisingly complex[0]. The editor needs to be able to read your inputs (mouse and keyboard) and represent those changes onscreen just like every other piece of software. It needs to repaint the screen with your changes and, depending on the complexity of the editor, load and implement a host of features (auto-complete? bracket matching? code indentation? code standards a la pep8? updates to source control?), with every single keypress and ideally in a matter of milliseconds. All of that takes memory and different editors make different sacrifices.

Of course the extent to which this happens is the whole reason there are different editors. If all you want to do is write text with zero features, then speed isn't an issue at all and you can just write directly in your terminal to a single file with pico or something.

[0] https://code.visualstudio.com/blogs/2017/02/08/syntax-highli...