[jonathanstrange]

Because any country that joins the EU agrees with the European Union Treaties in a legally binding way, and among other things these guarantee certain minimal democratic standards and human rights in the European Convention on Human Rights. The government of a country cannot just pick those parts of signed and ratified treaties they happen to like at one time or another and reap the benefits of membership without also fulfilling corresponding requirements and duties.

They would have to leave the EU first, which they are free to do at any time. Of course, then they would also not get any membership benefits.