As i said they 'kind' of tell you how much it is they are adding on at the same time they inform you of Insurance Premium Tax, breakdown cover, Motor Legal Protection etc etc
The only thing i can think of as to why people dont realise a lot of the time(I hear this while monitoring calls) is, at the end of a 10-15 minute call after you have already agreed to buy something, people are so anxious to hang up the phone and have a cup of tea they are not really listening.