Everyone is armed all the time and that’s normal, but to draw a weapon is an overt hostile act. A standoff therefore is a game of chicken because both want to kill each other and you want to draw first to have the highest chance of surviving, but even a bandit will hesitate to add a felony murder charge to their rap sheet. The whole town serves as witness when there is a pair of eyes behind every shuttered window. The hero always draws second, both demonstrating his superior skill and speed by defeating the opponent even at a disadvantage, and getting away with murder scot-free.
That’s what I used to think as a kid - westerns standoffs are an outgrowth of Old World duels, a formalized custom performed by traditionalists, even the baddest bandit secretly being a gentleman at heart. The showerthought here is that the order of events is also perfectly explained by taking legal considerations of justified self-defense into account.