Why does sitting in the passenger seat of a private car give the owner a polite feeling?

It is a private car, so it is important to pay attention to etiquette and politeness. The other person in the passenger seat can see your every move, and can also chat and communicate well with the owner, so that the owner will feel that you are A very qualified person sitting at the back will think you are superior like a leader, which will make the owner very uncomfortable.