Because in this day and age a lot of people don’t like confrontation, I’ve learnt this over the years in business that indeed you need to talk to people, but for some this is a difficult skill set.
Information is everything, and if you are not certain you are right, or if you even know your rights regardless it makes it very hard to have the confidence to have a direct conversation ‘on the spot’.
Mail has always allowed people the opportunity for a considered enquiry/answer.