> the richer parts have to give to the poorer

That's what's different monetary-policy-wise. California, for instance get's like 10 cents on the dollar back from its federal tax burden. Those 90 cents are used to shore up economically (and perhaps other kinds of) backward states like Mississippi. Nothing like this happened in the EU. Germany keeps a lions share of revenue and sends a tiny trickle back to the EU. This was never going to last long. Britain has probably only hastened the inevitable.