Because they did hours and hours of research, testing, usability sessions and found that it worked.
Why can't I ever find anyone who liked it then? Everyone in my family and non-geeky friends are struggling to find anything on the ribbon or are complaining it takes entirely too long to find something that was just fine living in the menu.
It seems that I am one of those rare people that love the ribbon (at least in Word and Powerpoint). I don't know exactly why I like it, but after using the new Word/Powerpoint for just a few minutes I never want to switch back to the old menu systems.