Obviously the situation today is different than it was in the late 18th century. We don't have a vast continent populated only by neolithic people who could be (relatively) easily exterminated so we could expand into their territory.
Our natural resources are still vast but not what they once were. People are accustomed to the privileges of a world power and are loathe to give that up, no matter how ruinous it may eventually be.
It's funny how those original 13 states assumed to take their place among the self governing nations of the world even though they knew they were but pipsqueaks compared to the might of the European nations. They knew that if they separated from Great Britain they would be a tiny, independent country. But so great was their thirst for freedom and self determination that they went ahead and did it anyway.
Like any situation, it's not all black or white as so many wish it were - black and white situations are easy to argue against and say "look - one part of your argument is wrong so therefore your ENTIRE argument is wrong".
There would be great difficulties in any union of states smaller than the 48 contiguous we have now. But the fact that so many are seriously considering it is a litmus test in itself of just how badly people chafe under the rule of a far off, indifferent autocracy with an agenda that is often 180 degrees apart from their local needs and wants.
