This is something I first noticed about a year ago, give or take. Like, I’ll say “the sky is purple” and someone will respond with “it’s blue?” Why do people do that?
It’s such a strange thing for me, because I’m used to question marks being used for questions, not statements. It feels like at some point, I accidentally fell into an alternate dimension where this is considered a normal use of punctuation.
I know English is a continually-evolving language, so things like this shouldn’t be unexpected. Even still, this development feels bizarre to me.
It’s trying to simulate tone over text. For situations like your example, the implied question is something like “why do you not know this?” Or “what are you talking about?”
Assume the person is giving you a quizzical facial expression on the other side of the screen, like they’re baffled by what you’ve said.