Because precise dates are used much more commonly contemporaneously than they are for historical purposes. This is so true that the year is commonly omitted, as it is assumed and understood by all parties without mention.

You don't need the comma when you write it this way. The comma in June 12, 2024, is there exactly because it's the wrong order.

It's basically "I wrote the date. Oops, forgot the year!"

Computers order it correctly in that format because that's the correct format. In the same way a computer will order any other correctly formatted numbers in the correct order - and incorrectly formatted numbers in the incorrect order - it shouldn't be surprising that they order correctly formatted dates in the correct order.

I figure it's because the year can be seen as an optional appendage if you're talking about dates from the current year. Like, I can say "that happened on May 5th," or "I'll be there June 18th," and you can reasonably assume I mean in 2024 unless I specify "June 18th, 2063."

Now, as for why you can say "I'm going on the 18th," but Americans don't say 18th of June, 2024, I haven't a clue. We really only seem to have logical explanations for the way we do things about half of the time.