Because voters decided there was a common interest, cultural identity, etc. and mutually agreed on political integration. Voters clearly do not want a unified Americas.
The purpose of a Republic is to be a stable entity that ensures the welfare of its citizens. It is not to have a single-minded obsession with global welfare at the expense of its own sustainability or the desires of its citizens.
> voters decided there was a common interest, cultural identity, etc. and mutually agreed on political integration.
I'm less confident that this was performed in either location due to direct democracy, and more because it made political sense and was expedient at the time that these locale enacted the governance structure.
In other words, it's not a one-and-done-forever type discussion, and things (clearly) evolve over time.
The purpose of a Republic is to be a stable entity that ensures the welfare of its citizens. It is not to have a single-minded obsession with global welfare at the expense of its own sustainability or the desires of its citizens.