This time around, they can't transform on their own. They always power down between vehicle mode and person mode. During this powered-down step, they can't move, so the transformer requires a human or a group of humans to help articulate their joints and flip their head up and stand them up and everything.
By the second movie (or maybe end of the first), whole pit-crews become trained to transform each of the individual transformers as quickly as possible, and there's a tense rivalry between the autobots' pit crews and the deceptacons' pit crews.
When they sell the toys, they tell kids "you can transform your favorites just like the crews do in the movies!"
