Most drones run dedicated code. When most OSS drone projects started, embedded Linux systems were still substantially more expensive than microcontrollers, and real-time control systems are probably harder to achieve with Linux than by hand.

Many hobbyist drones are even still based on old 8-bit AVRs because the first few popular projects were based on Arduino.

As far as I know every major project has moved on to 32-bit ARM, though, usually using STM32 Cortex M0 parts.

The Parrot AR.Drone is the only popular drone I know of running Linux as a primary control system.