It's not necessarily about speed of reaction, although that certainly plays a part.

Such software needs to react in real time though, if the task that's turning the steering wheel gets preempted in the middle of taking a curve on a cliff your self-driving car will become a self-flying car.

Such a system would be the equivalent of a driver that suddenly starts texting at all sorts of poorly chosen times.

It's more a matter of concurrency and blocking behavior. Gc blocks. A task you can't bound to an execution time blocks.