But wait, we already had a working mechanism to signal exactly this type of opting out[1] so let me rephrase the OP question: why does OpenAI get to be exempt from existing opt-out mechanisms and implement their own?
It certainly does seem as if they're trying to position themselves as a new standard against which content owners have to actively opt-out, and thus disregarding the already existing active opt-out signals. But that would mean that they don't actually care about privacy, and their opt-out signal is disengenuous! That can't be right, can it?? Surely everything they do is in good faith, just like every other corporation ever!
Anyway, the fact that they disregard existing privacy standards and rolled out their own privacy standard definitely gives me a lot of confidence that they will forever follow the privacy standards they themselves created!
Now excuse me, but I have to go get treatment for terminally metastasized sarcasm.