Because I did it! I'm guilty. I did the thing, I hit reply all to an email I was BCC'd on, and not even 2 minutes later, I see "Dude... you were BCC'd" I was embarrassed, and apologetic. Somehow we are still friends, but I felt like I "burnt" her a bit, and that is not something I wanted to feel again.
I think we've all been there, or at the least observed it happening. After a handful of research cycles, we observed tha the problem is a universal thing, and makes you question why the native email platforms dont further emphasize or flag you as a "hidden observer of the email". Just a single click path stands between you and exposing that.