Unless it processes images on the fly to do some sort of image-based steering or target recognition, an Arduino or comparable microcontroller should be beefy enough [1].
In mobile applications, power consumption of a RPi quickly becomes an issue.
Yes, an autonomous drone would have to process images, height data, map data, etc, so the compute of an RPi would still be relevant. Also, the drone does not have to max out the CPU all the time, it just may need intense compute in some situations, say, depending on the terrain. Larger drones, such as delivery drones, have motors with power consumption that dwarfs that of an RPi. For a a small, palm-sized drone it's of course untenable.
[1] https://github.com/ArduPilot/ardupilot