Because it gets horribly fucky when you now have to figure out if a date is actually formatted as MM-DD-YY or DD-MM-YY.
Surely we’ve all handled reading an expiration date before and have wondered if we’re eating something OK or has expired months ago because they chose the other format.
(Honestly, I think both formats are shit, and the only correct way to do dates with numbers only is YYYY-MM-DD. If not, then at least use letters for months, like 30 AUG 2023)
Then you’ve never bought imported food or never got food gifts overseas. Or never travelled to a country that used the format that you don’t use.
For example, 06/09/2023 could mean either you’re eating something that expires next month, or expired two months ago.