Germans do that and just don't seem to grasp why it is annoying which is weird for a federal state like Germany, because I bet they'd not be happy if I'd say Bavaria when I mean Germany.
... except the Bavarians, they'd be delighted. Probably.
Possibly to do with the fact that Bavaria didn't go out and conquer the rest of Germany and forced everyone to speak Bavarian.
If you speak German, you are German, if you speak English, you are English. That's probably the logic.
In short, the fact that England dominates the other countries makes such mistakes more likely to happen.
I get that it also makes them extra annoying, but that's how it is in mainland Europe.
Can't understand why British people do it, though, I mean, they should know better.