Can you explain why on a traffic light, red means stop and green means go? Why not the other way around?

Red is an aggravating colour psychologically. It's pretty universally used as a warning. Red lights in cars also mean "not ready to drive". Brake lights are also red for similar reason. "Seeing red."

Because it's arbitrary. Unlike a date format where the components have relative meaning to one another, can be sorted based on various criteria, and should smoothly integrate with other things.

As a US native let me clearly state that the US convention for writing dates is utterly cursed. Our usage of it makes even less sense than our continued refusal to adopt the metric system.