It's a game!!! Why do we let different people throw the dice? Can't a machine randomly generate numbers? Of course. We let humans throw the dice because it's more fun and we think it adds an element of luck/consistency/expected outcome. It's the only game in the casino where humans get to interact with "what comes next". Games have rules, whether written or not, and we expect a certain level of a social contract with regards to Craps. It's a sign of respect to honor these unwritten rules.
Why do we put money in the middle of the Monopoly board and collect it when landing in free parking? It's not in the rules but it makes it more fun. Someone at some point came up with that rule and added a new level of entertainment to an already popular game. It's the same with these "superstitions".