The thesis of vim is that editing is more fundamental to the act of writing than inputting text. The default mode, normal mode, is entirely shortcuts to do editing tasks better and easier. Moving around the document, and moving or deleting text are the primary concern.
What's more, as several other commenters have alluded to, vim thinks of editing as a language. There are verbs like cut or copy or paste, and there are nouns like [this] sentence or [this] word or one word backwards or from here until the letter j. Once you start building sentences of editing commands (e.g. copy from here to the 3rd next instance of the a line starting with 'Hello' namely <y3/^Hello>) the flexibility it offers is unparalleled. This is all done from your default context without fingers leaving the keyboard. Once you achieve fluency (admittedly not a trivial task) there is no context switching for any editing task, it becomes reflexive and subconscious.
Other stuff, like macros and registers, or easy access to regexes and ex commands help extend this, but the core reason why vim is still used today is this strength.
What's more, as several other commenters have alluded to, vim thinks of editing as a language. There are verbs like cut or copy or paste, and there are nouns like [this] sentence or [this] word or one word backwards or from here until the letter j. Once you start building sentences of editing commands (e.g. copy from here to the 3rd next instance of the a line starting with 'Hello' namely <y3/^Hello>) the flexibility it offers is unparalleled. This is all done from your default context without fingers leaving the keyboard. Once you achieve fluency (admittedly not a trivial task) there is no context switching for any editing task, it becomes reflexive and subconscious.
Other stuff, like macros and registers, or easy access to regexes and ex commands help extend this, but the core reason why vim is still used today is this strength.
I'd highly recommend another classic response, given here, elaborating on the nature of vim as a language for editing: https://stackoverflow.com/questions/1218390/what-is-your-mos...