helicopters also have one major advantage for manned applications: autorotation. there's at least a chance to survive loss of power in a helicopter. a quad with no power just falls.
I'm kind of surprised they use only 4. There are drones with 6 and 8 rotors, which can still fly if 1 fails, which they use in the film industry to fly heavier, more expensive cameras. There are also ways to get quads to fly on less than 4 rotors if one becomes damaged, although as a human I'm not sure I'd want to be in one doing it. Here's a Ted Talk with a live demo from 2013: https://www.youtube.com/watch?v=w2itwFJCgFQ
Autorotation is a great thing to have when your motor fails, but presumably multirotors would at least make up some reliability by having several motors and presumably being able to safety land when one motor fails.