Some people do so simply because they believe it is the right thing to do.
Similar motivations are in operation, I believe, on our adherence to cultural standards. For example, when I visited England for the first time, I found it very difficult to reverse the culturally ingrained principle of looking first left and then right before crossing the street. On the other hand, I also remember my first trip to the post office in Germany after I had arrived there for a year of university studies. To my American eyes, the post office window looked empty, and I walked up to it to buy stamps, only to be met by the disapproving stares of the twenty Germans who were in line and who were upset by my breach of good manners. I learned that in Germany, people line up to the side of the window, not straight back from it.