In this situation, why do you have to wait before the left turn?

You answered correctly that you have to wait because of the motorbike. According to German traffic rules, when making a left turn at an intersection, you must give way to oncoming vehicles going straight or turning right, including motorbikes. This is to prevent collisions by yielding to vehicles that have the right of way from the opposite direction.

Even if there are other road users such as cars or bicycles nearby, the primary concern when turning left is to ensure no oncoming traffic, like a motorbike, is approaching and has priority. This rule helps maintain safe and orderly traffic flow at intersections. So, always watch for oncoming vehicles and wait for them to pass before making a left turn.