There are RPGs in which you don’t play against other people but you can interact with them to sell items. In these games being able to generate vast amounts of in game wealth creates inflation for everybody and makes it so that normal players can’t interact with the economy. With regards to your other point about separating the player base it would create fragmentation and higher queues for matchmaking. Finally keep in mind that even in single player games people compete for speed run records or world first achievements.