An RPG needs isles of civilization amid defunct social order. That's how you get premises for quests, character motivations, etc. I don't see these conditions in 19th c. USA except if the game is based during the Civil War, which sounds pretty boring at first glance.
Disagrees with no arguments. Oh, ok.
I disagree you on two fronts.
A RPG doesn't need isles of civilisation amid defunct social order at all. It can have isles of defunct social order amid civilisation for one if we want to go that route, say a RPG concerning criminal underworld in a fairly functional society otherwise. It can have no defunct anything and can have a conflict of interest and interest of conflict between different factions. I could expand on this more but I think you get the idea, to be sure what you purpose is easiest and thus most common way to give player quests and character motivations, especially in fantasy RPG but it's by no means a necessity.
I also disagree that 19th century USA doesn't have isles of civilisation amid defunct social order, especially at frontiers, mid-west, south and northern borders, there is Texan war and Spanish-American war on top for two extremely easy settings.
Maybe make sure you understand what is written before writing in reply that you disagree (or agree)?
In an environment where social order is being enforced, there is no potential for situations that require combat to realistically occur. You need to have areas/spaces/environments where characters have to turn to violence in order to protect themselves.
So you always need a setting-provided excuse for the existance of areas like "wilderness", "dungeons", "infested sewers", "enchanted forests", "dark caverns", etc.
In the majority of RPGs such unsafe areas are the majority, and the safe areas are the minority, hence:
An RPG needs isles of civilization amid defunct social order