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.