Technically US states are sovereign too :) ...and really, if you looked at the US states as separate economies you'd see many glaring dichotomies that would make you wonder how it can be considered a single economic entity.
No, Europe's not a country, but it behaves like one for trade and currency purposes and has no internal tariffs. Nor can Germany (for example) impose a tariff separately from the EU. So in this context, the EU might as well be a country.
Technically the USA isn't a country but an agglomeration (federation) of states.
Originally the EU was fully called the European Economic Community, the goal was to form a single economy. Whether or not it is a single country is not an issue here, the OP spoke about 'economy', not about 'country'.
Does it absolutely have to be a country to be relevant? The EU share the same currency. I don't know for sure, but I can't quite think of a reason why economies absolutely have to be countries.
No, Europe's not a country, but it behaves like one for trade and currency purposes and has no internal tariffs. Nor can Germany (for example) impose a tariff separately from the EU. So in this context, the EU might as well be a country.