They don't want to become like USA, that is the easiest argument. We see how USA went the slippery slope of removing states rights and expanding federal power,

They don't want to become like USA, that is the easiest argument. We see how USA went the slippery slope of removing states rights and expanding federal power, people in the EU don't want that. To prevent that from happening there needs to be a resistance to EU expanding its power, and the most effective way countries can do that is to threaten to leave.