As RC vehicles {car|helicopter|plane} get larger they reach a certain mass where liquid fuel - usually a petroleum - is preferable to battery. Why is this not an alternative for drones?
You're looking at a fixed wing aircraft though with one of those. What's made the quadcopter drone possible is the high torque and instantaneous throttle response of small electric motors, combined with fast control electronics. The conventional way to fly a small helicopter is just like a full sized helicopter, with a main rotor and a mechanical swash plate.
With the right control electronics, these can easily be made into drones, but people tend to not like them as much anymore because they're mechanically complex and finicky. The electric motors are simple, and when one goes bad you just drop in a new one.
I think that’s a more impossible goal than increasing power density in batteries is.
This guy tries to make gasoline powered generator for large drone.
This is hard to do with petrol engines, without costing a lot of extra weight or mechanical complexity, in a system where any single failure means a crash
There's differential based continous variable transmission that varies according to resistive torque on a control shaft: https://makezine.com/2012/08/10/lego-continuously-variable-t...
This might enable electronic control witout using any motors just one simple coil per propeller. Energy generated through this way of control could be stored to power onboard devices.
Given the line of sight limitation for drones, gas power does not add much value. Batteries are usually sufficient to map/photograph the area within range of the operator. Given this legal limitation, it is often more economical to hire a small plane like a Cessna with an avionics package and a 400 mi range.
They exist, but they are expensive. https://www.youtube.com/watch?v=wS0Hr2lw604