I should have clarified that it is how dates are pronounced in American English. British people will say “it’s the 21st of July, 2021”.
I don’t think the US resisting things is so mysterious. The US is a large country that largely defines its own culture (and exports that culture to the world). The average american isn’t going to switch systems unless they have a VERY good reason to. For the fields where it matters (ex. the sciences) they’ve largely already switched.
The US always seem to resist sensible things when it comes to units and formats: it is a mystery to me why they are so attached to imperial units, when the rest of the world sans Liberia and Myanmar are using metric units (and Myanmar planned to leave this shameful club before the recent coup).