And drones don't scale linearly. Small unmanned "toy" drones can get away with RPM control of their props to maintain level flight. RPM control is easy with brushless DC motors and it's mechanically simple. But drones big enough to carry people can't do that (in general) because you cannot change RPM on a big prop nearly as quickly as you can on a small prop, because of inertia. So big props often need collective pitch just like helicopters. Collective pitch is far more mechanically complicated than RPM control and it thus presents more avenues for failure.
The good news is that multirotor drones (even big ones) usually don't need cyclic pitch which is even more complicated than collective pitch, so that makes them less complicated than helicopters, but big drones are still not as mechanically simple as toy drones.
Can't you build an array of smaller (say brushless DC motor) propellers to achieve the same thing? I could have sworn I saw a guy on Youtube do that and hover around his backyard in a home-made frankendrone comprised of a ton of props
You can, and hobbyists have done exactly that. But such aircraft are extremely inefficient and only have enough battery power for a few minutes.
It's kind of like that guy who tied a bunch of little helium balloons to his lawn chair and went flying. It sort of works but it's pointless and dangerous.
Yes, you can certainly do this. But it won't be as efficient because in general large propellers are more efficient than small ones. So your batteries won't last as long.
