The European single market is essentially the largest international free trade agreement, going far further and deeper than any other by eliminating as many barriers to both trade and individual freedom between states. In order to support that and provide a level playing field, there needs to be common set of rules on things like regulation, worker’s rights, state funding of industry and so on. And in order to administer those, you need a level of bureaucracy. Quite quickly, you’ll find you’ve recreated the EU.
It’s totally reasonable to argue for the EU to take a particular direction - like closer integration, or more distant cooperation, or less bureaucracy or whatever. But the arguments in the Brexit case in particular often seem to fall apart on interrogation of the details.