UK Brexit, questions from an American

[Dystopia]

We've got states "holding other states back".  I'd be awfully confused if some rich state wanted to leave the US because it had poor states.  II know that there are fundamental differences between us states and the eu states...still, seems strange.  Why not bring them up..and then profit/prosper together to an even greater degree?  Wasn't that the idea in the first place?

That was the original idea... But... It's not working that well. Poorer states continue poorer and the EU organizations are dominated by the most powerful (economically and politically) states like Germany, France, etc.

There is also a sentiment of rebellion towards the EU extending its political powers in the last decades and many people want sovereignty back.