…as countries enter these groups voluntarily for their mutual benefit.

You are deeply wrong here. There is no voluntarism after Maastricht Treaty, ie. unofficial EU Constitution. By the treaty EU bureaucracy got too much power, and started to enforce everything. That is exactly the reason which UKIP used to start a campaign to leave aspiring Empire. Please note that I always say aspiring Empire for EU, because it cannot be an Empire without its own army. That is why Macron and Merkel mention the idea more often lately.