This is unnecessarily flippant, trivializing, and reductive.
The upper classes had the time and position to refine manners. I think one mistake people make is to think manners are arbitrary nonsense. But manners, when fitting, honor the self and others with conduct that suits the dignity of the human person and functions as a sign of that dignity. You cannot tell me that a man hunched over a table cramming food down his throat gaping at a television is no different than one who eats according to the above custom of etiquette.
I’m not one for stiff artifice especially when slavishly applied, but I don’t think manners as such are arbitrary. That nobody cares would explain why so many people look like slobs and behave like boors.
If we begin with human nature and then view the virtues as perfections that actualize the fullness of that nature, then it becomes clearer that some behavior is more fitting and honored better by certain practices.