The same reason we use all the other Vim keyboard shortcuts - to keep the hands on the home row!

I can ctrl-f way the f faster than someone who has to move their right hand down or over to the pgdn key.

I once read a great explanation about programming language wars. The author described a ladder of awareness, such that some languages were lower on the ladder than others. The idea was that programmers who knew languages at or below their level on the ladder could easily see the people below them and recognize that their languages were inferior, but they were unable to see the superiority of the languages above them. Of course the superiority of a language is somewhat subjective and largely dependent upon situation, but the concept is quite valid.

Do you use the mouse to click menus, or do you occasionally "cheat" and use a ctrl-c, ctrl-v, ctrl-s, whatever? If you use those keyboard shortcuts, why not other useful ones?