Reusing one's functionality is amazing, but not always the best solution.

The example you gave is exactly this: imagine if your web-browser would create new windows instead of tabs. Why? Well, it would just let OS manage the tabs, afterall that is the main functionality of the Operating System.

Hardly everyone wants their vim to create a bunch of tmux windows messing up their workflow: constantly changing IDs of tmux windows; slow navigation between vim files as user now has to open ID 15 (tmux); mixed order... Oh that sounds as bad as seeing all your open websites in Meta/Alt+Tab.

So please, do not speak for everyone with arguments "vim shouldn't have its own window functionality".