There is more that makes the EU a country, than makes the EU not a country.
Citizenship. The first and most obvious requirement for a country is to have citizens with citizenship. Does the EU have citizens and does it have citizenship?
Yes! This can be seen in the legal status of how EU Citizens are treated
by many countries - there are rules governing all the citizens of the EU, not constituent countries. EU citizenship
gives certain rights.
Tax rules. Does the EU have a common tax policy? Yes!
There is a minimum normal VAT rate of 15% and a minimum reduced VAT rate of 5%, with exemptions allowed for historic reasons and through treaties.
Import duty is also paid directly to the EU.
Central budget. Does the EU have a central budget?
Yes. It is collected and paid for by each member as well as paid for by import duties direct to the EU.
Is the EU a customs union?
Again yes. There are countries that don't even have a harmonious customs border, but the EU does.
Foreign policy. Surely the EU doesn't have a foreign policy. Actually -
yes it does. And
a foreign minister.
Does the EU have anything else normally reserved to countries?
How about membership to the WTO! And has "
enhanced observer status" in the United Nations! (other non-member states include Palestine)
There is nothing that makes the EU *not* a country, except that we don't call it one by convention, and it doesn't want to be one.