Motorways are meant to be safe for drivers who wish to maintain steady progress at 80km/hr – 120km/hr for long distances and not have to worry about slow moving hazards on route, such as cyclists, pedestrians and inexperienced drivers. Could you imagine a road full of cars tearing along at 120km/hr, mixed in with learner drivers struggling along at 30km/hr and fumbling with the steering wheel? It just wouldn’t work.
In any case I don’t see why learners would need or want to be on the motorway. The motorway is for long distance journeys. An L-driver isn’t going to learn much sitting on a motorway for 50 miles. They need to be driving around city streets where the traffic moves slowly and they’re exposed to far more while under the supervision of someone qualified. There are dual carriageways for them to learn merging and lane discipline.