Why is the United Kingdom often considered a country, but the European Union isn't?

United Kingdom is often considered a country

The United Kingdom of Great Britain and Northern Ireland, commonly known as the United Kingdom (UK) or Britain,[m] is a country in Northwestern Europe, off the coast of the continental mainland.

United Kingdom

The United Kingdom of Great Britain and Northern Ireland (UK) is an island country that sits north-west of mainland Europe

The Commonwealth

However the European Union, which also consists of many countries, and which has most of the same government institutions any country has

the European Parliament,

the European Council (of heads of state or government),

the Council of the European Union (of member state ministers, a council for each area of responsibility),

the European Commission,

the Court of Justice of the European Union,

the European Central Bank and

the European Court of Auditors

is actually considered,

"a supranational political and economic union of 27 member states that are located primarily in Europe"

European Union

is there any substantial difference for the European Union not to be considered a country? Someone could argue their members countries have their own constitutions, and that this would turn the Union "not a country" (I believe Wales doesn't have a Constitution, for example), but this also happens with United States states, or with Argentina provinces, which also have constitutions, and their parent government body (United States and Argentina) are still considered countries

Why is the United Kingdom often considered a country, but the European Union isn't?

One important sign of sovereignty is the ability to conduct foreign relations. It's indicative enough that a colonial war was fought over it (see Treaty of Wuchale).

From there it is easy to see why the UK is considered a country while the European Union is not. The UK's constituents all conduct foreign affairs together. Scotland cannot for example choose to remain part of the EU while the rest of the UK leaves. On the other hand, the EU does not conduct foreign policy as one; we get things like Hungary continuing to import Russian gas while most of the rest of the EU are critical. When France deploys its arms to Ivory Coast, it's acting on its own capacity and not the EU's. Etc.

In the same way, the states of the US are not countries.

One basic reason is that the EU does not claim to be a country. So far as I know, no entities that have not claimed to be countries have ever been recognised as such.

You have listed some attributes of a country which both the UK and EU exhibit. But there are other attributes which the EU does not exhibit:

For many of these, the member states chose to act in concert with each other so that there is almost an appearance of a single country. But crucially, they are doing it voluntarily. They can and do break rank whenever they want. Whereas for example if UK goes to war with someone, Scotland cannot simply decide to sit it out.

I think USA is a better comparison, because it is more similar. There are member states with their own quite different laws, their own tax systems, even their own armies. There are fewer differences, and I think they are more informative. But you're asking specifically about UK, so perhaps USA should be a separate question.

The usual formula is that a country has a state with a monopoly on a people, a territory, and an army.

There is generally also a common language, at least amongst all those who govern or administer the country on a daily basis, and a capital city.

The EU does not have any of these things, and has never really been intended to be a country of its own. Instead, the intention has been to regulate key aspects of the relationship between its member states through diplomatic institutions, which otherwise remain as separate countries.

In addition to all good answers, there is also the element of representation. In the UK, one head of state and one prime minister represent the whole of the UK. Although Wales, Scotland and Northern Ireland have their own governing bodies and representatives, they usually do not speak on behalf of the UK. The same applies to the United States.

In the case of the EU, it is basically a collection of sovereign nations to demonstrate a unified front mainly from a trade perspective but also extending to security and other areas.