Because as others have already explained, that's not what that phrasing suggests. "If you were/you would" suggests that it's a purely theoretical perspective and thus strongly implies "you aren't".

"If you were British..." and "If you are British..." have very different subtext.

No they don't. No it doesn't.

Nuance of the English language, but, "If you were British, you would know where York is", suggests I'm not British, and implies I don't know where York is.

That specific form of the sentence has negative connotations and might be considered divisive in (British) English.

It's essentially a challenge to my honesty, and had you said "if you are" rather than "if you were" it'd be a more amiable challenge, implying I should respond with proof that I know where York is, and not suggest I'm being dishonest.

As I said, it's divisive, but to some may be considered offensive too.

Just one word makes all of the difference.

If you want to suggest I would know where York is by virtue of being British, you might say;

"You will know where York is, because you're British" or "You must know where York is, you're British".

No. It does not suggest that. I use British English and that's not at all what it means. Maybe to those that don't know how to use it correctly and are seeking emotional connotations.

The statement is at most doubting whether you are British, but not dismissing or claiming the opposite.

>but to some may be considered offensive too.

It's offensive that someone might claim to be from somewhere and not understand a very common piece of knowledge from that place. Or at least I might want to claim it to be.