The one you grew up with tends to be your preference. But the usual reason I give for MM-DD-YYYY is it matches how it's said.

December 25th, 2026

12-25-2026

I also know there's no point in arguing about this because, again, my first sentence. It's just team sports stuff, as usual. The only reason why you would bother to argue is if you were trying to convince people to use a standard. But as you pointed out, ISO 8601 already exists. That is the agreed upon standard already. But no one uses it, since we have our preferred systems of date format already ingrained.