Earlier quoted context omitted.
The EU in many ways acts like one big economy due to its customs union, freedom of movement of workers, capital, goods and services, extensive harmonised regulations, partially harmonised taxation, wealth transfers between members, a common currency to a substantial part of it, etc. It is a lot more tightly integrated than some mere 28 member free trade area would be, although still not as tightly integrated as say t…
States can't leave the USA countries can leave the EU, which is a massive difference. NAFTA and EU are not countries they are an agreement between countries.
EU member states have only been able to unilaterally leave the EU since the Treaty of Lisbon came into force in December 2009. Prior to that, there was no legal provision for a member state to leave without a treaty amendment. So the situations between the EU and the US are not as dissimilar as you suggest.