The EU has never been "just a free trade zone", though.

It has always had a political element.

To quote a 1963 Court of Justice judgment:

"The Community constitutes a new legal order of international law for the benefit of which the states have limited their sovereign rights, albeit within limited fields and the subjects of which comprise not only member states but also their nationals. Independently of the legislation of member states, Community law therefore not only imposes obligations on individuals but is also intended to confer upon them rights which become part of their legal heritage. These rights arise not only where they are expressly granted by the treaty, but also by reason of obligations which the treaty imposes in a clearly defined way upon individuals as well as upon the member states and upon the institutions of the Community. "

The trade union, although critical to our modern day understanding of the EU, was actually not the primary goal, it was all about politics, primarily about maintaining peaceful diplomatic relationships between countries. It's just that doing business together is one of the more effective means to be friendly to each other.