Because the operators at the call centre work from a standard script. They don't have technical skills; that's left to the guys on the road. Their job is to get as much information as possible to the patrol who is coming to you.
Bear in mind that 99% of the calls they take are to drivers who have no technical competence at all, and who could well have run out of fuel without knowing. It makes sense to ensure in that case that the attending patrol has a can with him.
Chris