An old-schooler would be the one asking, "why do I need a mouse?"
Anyway, it sounds like you're asking, "Why are keyboard shortcuts faster than moving a mouse to a location and then clicking [and potentially having to move to another location, such as the menu that just dropped down, and clicking again]?"
You forgot even more inefficiency: Lifting your hand off the keyboard and moving it to the mouse, [insert your above steps], then moving back to the home row.
No problem at all if you use a TrackPoint. I don't even have a mouse, I use the ThinkPad USB keyboard at work which includes a TrackPoint. I know some people dislike them with a passion, and they don't look fashionable, but I can't use a keyboard without one any more.
Actually, this is also a point worth stressing for people without Vim-experience: Searching is a "first-class citizen" (as the FP-guys put it with functions).
It's only ever one to two keystrokes away. You press '/', enter your phrase, and press ENTER. Stepping is done with 'n' and 'N'. There really are no convoluted hotkeys for this - which means that you do it much more liberally once you're accustomed to it.
Also there's '*' and '#' to quickly find the next/previous positions of the word your cursor is currentl located on. The same thing applies there.
That's what you do with a mouse though. With a keyboard, you can move by semantic units (like a paragraph or function), use fuzzy search to jump to a specific function, etc.
I didn't forget. I just didn't feel like enumerating what I think are obvious and commonly accepted facts :).
It really comes down to thought->muscle memory of keyboard shortcuts, especially those which don't require moving hands away from the home keys, vs the activity of reaching over, taking the mouse, driving it to a certain location on the screen, and then clicking. It should be obvious which will be faster.
Anyway, it sounds like you're asking, "Why are keyboard shortcuts faster than moving a mouse to a location and then clicking [and potentially having to move to another location, such as the menu that just dropped down, and clicking again]?"
Does this really need an answer?