Why is the national level different to the continental and regional levels?

You can apply that same logic to counties: Greater London is very different than Leicestershire. Why should Leicestershire apply laws that are designed to fit London workers? Why can't they make their own laws?

Forget counties, why not neighbourhoods? And so on.

I'm obviously not serious about this, but the logic doesn't hold just because your nationality is different. The average briton has far more in common with the average french, than the average londoner has in common with the average belfaster.