If helicopters are more efficient, what are the advantages of drones?

I would look at it this way.

A helicopter has a complex mechanism in the hub that rotates the blades up and down as the rotor rotates

https://aviation.stackexchange.com/questions/73923/how-do-a-...

I bought a quality RC helicopter from a hobby shop that I was a part owner of and really didn't have a lot of fun with it because I was maintaining it about as much as I was flying it.

The control problem for both a helicopter and a quadricopter is tricky because it is fundamentally nonlinear (if you want to move in direction x the first thing you do makes you move a little bit in direction -x) but the mechanism of an electric quadricopter is much simpler than a helicopter because you just need PCM control of four motors rather than a swashplate mechanism that is trying to destroy itself with friction, vibrations, etc.

The helicopter was always a 'stillborn' technology that didn't improve on the same curve that airplanes improved on. Quadricopters started out small and are scaling up and are improving faster than helicopters so at some point the curves cross and quadricopters win.

My guesses would be that the quadcopter doesn't need to control pitch of the rotors, nor do the rotors need to be balanced as perfectly. I believe an electric helicopter would still need a transmission, which would be a potential point of failure (and expensive). Different noise profile too (likely preferable for the quadcopter due to smaller blades that can be designed to work with with fixed pitch).

There are a few experimental human transport EV quadcopters out there.

Redundancy. Multirotors have - wait for it - multiple rotors. An octocopter or hexacopter can keep flying when one of its props or motors break.[0]

This is why you see Hollywood cameras carried by octocopters.

[0] https://www.dronetrest.com/t/what-happens-when-you-have-a-mo...

The tradeoff is between mechanical versus software complexity.