They could. They just need variable pitch (aka collective) on all the blades. And this needs to be actuated by a different power system so that it remains operational when the main power stops. Helicopters use hydraulics, or even manual power.

Autorotation is where helicopters get their speed/altitude restrictions when carrying passengers. They generally are not allowed to hover at low altitude because from there they cannot safely autorotate onto the ground. If drones want to carry people, or carry heavy things over people, then they will need to develop some sort of plan. Total engine failure should never result in the total loss of the aircraft.

Over a sufficiently long and thorough testing period, I think x8 design multirotors with eight or more motors/props will prove to have a statistical rate of motor failure much lower than the incidence of single engine failure in small to mid size internal combustion engine and turbine helicopters.

It would have to monumentally better before I'd get into one that didn't have a backup plan. The motors could be 100% reliable, but the possibility of loosing power due to maintenance or control issues remains. A perfectly good motor is useless if the cable feeding it electricity shorts out.