I don’t agree. The m/d/y vs. d/m/y thing can cause major headaches. If you see “5/4/06” and you know it’s American, it’s not a problem. But if there’s any doubt that it could be American, could be European, you’re either hosed, or you have to track down its origin.
As to why, you have to understand that, believe it or not, there was a time, many many years ago, when “global communication” didn’t exist. No, I don’t mean just back before there was an Internet, I mean even further back. Before long-distance phones. Before radio, even. Back to when, to get a message from one place to another, some person had to actually physically travel the distance. And back even before that. Way back then, different areas of the world developed independently and just developed different customs, that’s all. Americans all grew up with their m/d/y, and we all know how impossible it is to get Americans to change anything unless there’s clear self-interest in it. And Europeans, why should they change just to match those silly Americans, especially since d/m/y makes so much more sense?
So we live with the differences.