No I think it’s fine and my comment wasn’t sarcastic. US states really are similar to European countries, they are highly autonomous and have different laws and cultures. It could absolutely be the case that one state uses m/d/y and another uses d/m/y. We have states where you drive on the wrong side of the road (well, territories) and states where you aren’t allowed to turn right on a red light, and states where people call all soda pop “coke”, etc.

I think it really boils down to cultural homogenization and borders. Prior to the eurozone, Europeans couldn’t just walk across the border and live in another country. But the US has had that for its entire history. So perhaps cultural practices in the states homogenized earlier than in Europe; also language.