With robots a lot depends on the scope of movement and speed, and whether or not it interacts with environment/people. For some applications the controller is already dedicated hardware on the joint module anyway with some sophistication, connected to an CAN (or etherCAT) bus or something like that - so no OS is the tightest loop - I could see the high level control working on a RT linux or whatever if you wanted too, lots of tradeoffs. Mainly though it's the same argument, you probably don't want a complex OS involved in the lowest level/finest time tick updates. Hell some of the encoders are spewing enough data you probably end up with the first thing it hits being an ASIC anyway, then a MCU dealing with control updates/fusion etc., then a higher level system for planning.