Likewise, there's an entire country currently undergoing an invasion being fought not only by their regular army but by civilian volunteers too. And sure, such an event happening in the US is unlikely given our particular neighbors, but then again even up to the day of the invasion various analysts were sure it would never happen so predicting the future can be a bit difficult.
We don't live in a post conflict world, and our country isn't immune to bad actors.
The fact that many, many people have thought exactly as you do. That they had reached the pinnacle of governance to the point that they would never collapse or need to defend themselves. A significant portion of the population figured WW1 was the "war to end all wars" merely on the premise that the war was so great, like nothing they had ever seen before. We are seeing unprecedented peace but I see no reason to think that historical peace == future peace.
If you want a more "objective" answer... Conflicts arise far too often to suggest one of them won't turn into a scenario that allows for occupation or tyranny. How many people seriously predicted Ukraine would be invaded?
If you truly think "most countries are past that point" - which countries are you referring to and how long do you think they will last/remain stable? How would one come to this estimate?
Honestly, to me that number isn't nearly as impressive enough to justify surrendering one of your only practical defenses against a tyrannical regime.
This takes your chances of dying from a terrorist attack from 0.0000001% to 0.000000066% ?