Some people, specially women, receive a consistent share of harassment from some countries (you can see it on sites like Interpals, where you can block countries, or you can ask women on here), so they dismiss immediately people from those countries. Some people from those countries respond to this by changing their nationality, as they can trick someone with a beginner english level, who will not spot them as easily. This is one reason.

I've seen loads of people like this. As a native English speaker it's really easy to spot a fake, but for people that are learning it's probably difficult enough, especially some of the better ones.

THIS IS MADNESS !!!!!!

I've encountered a few different people in chatrooms that claimed to be native English speakers from Australia, the US or England. But they were lying.

Sometimes there are clues in their grammar. Even if their grammar isn't terrible there are still some subtle things that can be noticed. On some occassions, I've asked them cultural questions related to local celebrities, music culture, showed them pictures and asked if they knew who or what it was, or I spoke to them in slang. And then it became clear that they were lying.

I don't really understand why people do this. It's somewhat amusing but annoying. Especially when they try to argue with me to convince me, but I prove that they are lying.

I have not encountered anyone like that yet, but I can see how that could get frustrating, since I myself am a native speaker! They may just be confident in their ability to speak that they consider themselves native.