Because it doesn't give that person a chance to respond, ask questions or just look somebody in the eye. Some people in some situations may accept being made redundant/getting the sack as fair enough but some people in other situations may like the chance to give their view about events or find out more about it.
The longer a person has been working somewhere the more they feel like their view matters or or that possibly they should have kept their job over somebody else who did and find out why they didn't, this is why it is insulting to people, because they feel like they have given a lot and not been given any dignity.
Now maybe they deserved to be insulted, maybe they were lazy, theiving, lying little so and so's who were sleeping with the boss for 22 years, who knows, but fact of the matter is insulted they were.