It's not easier. They are independent nations, sort of at least. The problem is how does the money/wealth get into a tax haven in the first place? That's where the city comes in! You cannot just fly with millions in cash (still happens too) you need a way to move money there smartly to bypass the FCA & Co. Once the money has left EU jurisdiction it's too late!
All those tax havens have effectively sponsoring major eu nations to be able to function. Just get France Spain Germany and UK to present them with an offer they can't refuse.
France did that with Monaco in 1963: Charles DeGaulle blockaded the city until they signed a tax treaty. It didn't eliminate Monaco's tax-haven status, but did limit it in various ways, especially as regards French citizens and companies.
