You'd be correct if people used editors for opening only source code files. The problem is that people usually open data files too which can be not only larger

You'd be correct if people used editors for opening only source code files. The problem is that people usually open data files too which can be not only larger than L3 cache, but larger the entire system memory. The magic of a good editor like Vim is the capability to handle such files. The other problem with your comment is the support for Undo operation. Even if you use a flat array, you need a more sophisticated data structure for storing previous changes. Storing a separate array for every single change is not an option.