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)
When you say "don't store dates as a string" what you're really saying is "wait for someone else to solve the problem and release a library, then use that library". That seems to be what the majority of the industry does (I'm a Java coder myself and joda is a lifesaver in that regard) but my point is that this problem is hard. Date and time stamps are a subtly difficult part of the average API monkey's daily work.