If you don't mandate implementation, you don't have a level playing field; you have a race to the bottom on everything from tax levels to working laws to product standards. Game theoretically, everybody loses in a race to the bottom.
There would be no point in an EU that couldn't mandate laws.
The EU does not control tax law, yet apparently lots of people believe it still has a point. The EU cannot mandate tax or worker rights outside of the EU either. So the only way to stop a "race to the bottom" is to become entirely protectionist and go full North Korea.
Suffice it to say, the EU could have taken many structures that are not the one it has now.
There would be no point in an EU that couldn't mandate laws.