VIM works in a certain way. Once you understand it, the mechanism for exiting fits in just with the rest of the patterns. Why should they create a special case for exiting versus other operations? "Why is it so hard to search? Use ctrl-f instead of /" "Why is it so hard to cut and paste?" "why is it so hard to move the cursor?" You'd have a different editor if you changed the keybindings.
Vim's defining feature is the keybinding grammar, so it would no longer be Vim if you changed it.
vi's defining characteristic, which vim inherits, is that it is _modal_. That means that it has modes, like input mode (where what you type is added to the file), and command mode (where what you type are commands to the editor).
Modal editing is fantastic: it means you don't need a meta key, or multiple modifier key combinations to do things (for which your wrists will thank you!). It frees up keybindings for different purposes, which is partly how modal editing ends up needing fewer modifier key combinations.
Vim's defining feature is the keybinding grammar, so it would no longer be Vim if you changed it.