I can’t be ‘aware’ of this, because I disagree that it is. That’s only your personal opinion, even if you are desperately attempting to pass it off as some law of nature.
Hint: ethnicity does not mean ‘racial bloodline’. It includes culture in all its manifold manifestations.

You seem hugely confused. I am ethnically Jewish. That is a simple fact. Neither I nor even you can change this.

“If you’re an atheist, then why are you disputing theology?”

I grew up in a religious household. I know about these things at least as much as you do.

“You seem confused, when the answers to these questions are fairly simple.”

Then how come you manage to post such nonsense about the topic?

I agree. Especially when conversion is to one of the “replacement” theologies like Christianity and Islam. But they’re still Jews.

My husband is the Jewish son of Jewish parents, married to a Jewish wife born to Jewish parents, and is the father of a Jewish daughter and a Jewish son. The difference between my husband (and probably Tarquin) and this Argove is that my husband has never converted to anything else.

She made a positive decision to change her religion. Being an atheist is a matter of private belief, whereas a formal conversion to a different religion is a public rejection of the first religion (no matter what convolutions the Jews for Jesus oxymoronites try to conjure up), and, in the case of Judaism, a rejection of an ethnicity, a people, and our history.

When the lady in my congregation whom I mentioned above became Jewish, she became, not only religiously Jewish, but joined us as a member of our people as well, with all of the good and bad which comes with that. Our history became her history, our life, her life, just as Ruth joined Naomi.

In the same way, I think, for most Jews, those who convert to other religions do more than just accept new religious beliefs. They publicly reject all of us, four thousand years of history, family, and worst of all, we lose their children. It is really hard to accept, even when you believe that the convert is sincere in their religious conviction, and not just converting for the express purpose of rejecting us, and using what they know about Judaism and Jews as a club against us.

Why is it?

If you get to make up what a Jew is, then Joseph gets to make up what a Jewish Christian is. If you’re an atheist, then why are you disputing theology? You seem confused, when the answers to these questions are fairly simple.