Navigation / attitude control needs to be real time. Even if there's a microcontroller at the device, the guidance system receives telemetry and then sends commands. If the send is arbitrarily delayed then the resulting motion won't be what the system desired. The microcontroller can't do the complex calculations on its own because it doesn't have all the telemetry from multiple sensors.

Also RTOS tend to have a lot of verification work applied so that you can be confident some facilities just won't fail.