It certainly seems implausible that our physical brains would behave as if they experienced things without ever having any kind of input from that process, but not impossible.

But the point of the idea of p-zombies isn't to describe something that plausibly exists. It's thought experiment to highlight the hard problem of consciousness, by demonstrating how the notion of experience is separable in our conception from any physical process we can imagine. That is, regardless of how our brain physically comes to behave as if it were conscious, we can still conceive of a version of it that does not actually experience consciousness.

This is because, at least to me, conscious experience is something entirely unlike any physical process. I can conceive of it being the result of a physical process, but nonetheless, my conception of that experience is still of a separate thing which happens as a result of that process. And as such, I can always imagine any such physical process occurring without the corresponding experience.

To say that conscious experience "is" a physical process, is to my conception, a category error.