Because Vim is like programming text. You have motion commands, you have operators that act on text motion command moves over, and you can specify how many time to repeat the operator. You have programming specific motion commands, so called text objects. This is incredibly powerful. You learn to move around efficiently and you multiply your power, because now you can also make changes, delete, etc using the newly learned motion command.
General form of the command in Vim is: nOm meaning repeat n times operator O acting over motion command m. There are motion commands to move by letters, words, sentences, paragraphs, code blocks, XML/HTML tags etc. There are delete, change, insert, etc operators.
So for example: 3dw says 3 times (d)elete (w)ord (i.e. delete 3 words). di{ says delete everything inside code block.
Vim is also modal, there is distinct mode for editing by issuing commands, and there is insert (also replace, visual etc) mode(s) which is what all other editors have as the only mode. This makes it incredibly powerful, and it is possible to make large sweeping changes with very few key strokes.
Also, vim discourages mouse use. The context switch of your arm moving to the mouse is like CPU doing a cache miss and going to disk to fetch data. It's a huge distraction and if you add time to actually find the mouse pointer, select something etc, it really slows you down compared to Vim.
So, I still (since 1993) use Vim because it makes me really productive and I can edit text really fast with it. With that said, Vim is not an IDE (even though with language server and background tasks support it can be made into surrogate IDE). So, the best of both worlds is to have a good IDE with good Vim support. Jetbrains products have solid Vim support (much much better than say VS Code), so if you need to work with verbose language or work with unfamiliar libraries, IDEs are still useful.
However, I also love the ability to work in plain Vim when I work on code base I know really well and don't need IDE type of help. Vim is also great at working though SSH remotely, tmux etc.