You experience your own consciousness - your own model of your self, time, and the world as perceived through your physical sensory apparatus. This could give you a probability of 100% certainty of your own consciousness. You're a good skeptic, though, and after much consideration, you decide that, despite absence of evidence to the contrary, you'll allow for 2% uncertainty, since you might be a simulation specifically designed to "feel" conscious, or some other bizarre circumstance.

Knowing this, you compare your own experience with reports by others, and find that, despite some startling variety in social and cultural practice, humans all more or less go through life experiencing the world in a way that more or less maps to your own experiences. You find that even Helen Keller, despite her tragic disability, wrote about experiences which you can simulate for yourself. You conclude that if you somehow swapped places with her, she would be able to map the sensory input of your physical sensors to her own experience of the world, and vice versa.

This leads you to think that our physical brains are performing a process that models the world, and it does so consistently. After reading up on people's experiences, you also learn that our subjective experiences are constructed, moment by moment, by a combination of these world models, real-time stimulation, abstract feedback loops of conscious and unconscious thinking.

The more you read, the more evidence you have of this strange loop being the default case for every instance of a human having a brain and being alive.

The Bayesian probability that you are conscious, because of your brain, given all available evidence, approaches 100% certainty. You conclude your brain is more or less the same as anyone else's brain, broadly speaking, and this is supported by the evidence provided by a vast majority of accounts from other similarly brained individuals through all of human history.

Since your brain doesn't have a particular difference upon which to pin your experience of consciousness, and the evidence doesn't speak to any need for explanation, Occam's razor leads you to the conclusion that the simplest explanation is also the best. The living human brain is necessary and sufficient for consciousness, and consciousness is the default case for any living human brain.

The posterior probability that any given human (with a "normal" living brain) is conscious approaches 100% certainty, unless you can specifically provide evidence to the contrary. Saying "but what if p-zombies exist" makes for a diverting thought experiment, but it's rationally equivalent to saying "but what if little invisible unicorns are the ones actually experiencing things" or "what if we're all in the Matrix and it's a simulation" or "what if we're just an oddly persistent Boltzmann brain in an energetic nebula somewhere in the universe."

Without evidence, p-zombies are a plot device, not a legitimate rational launching off point for theorizing about anything serious.

Humans are conscious. We have neural correlates, endless recorded evidence, all sorts of second hand reporting which can compare and contrast our own first hand experiences and arrive at rational conclusions. Insisting on some arbitrary threshold of double blind, first hand, objective replicable evidence is not necessary, and even a bit shortsighted and silly, since the thing we are talking about cannot be directly shared or communicated. At some point, we'll be able to map engrams and share conscious experience directly between brains using BCI, and the translation layer between individuals will be studied, and we'll have chains of double blinded, replicable experiments that give us visibility into the algorithms of conscious experience.

Without direct interfaces to the brain and a robust knowledge of neural operation, we're left with tools of abstract reasoning. There's no good reason for p-zombies - they cannot exist, given the evidence, so we'd be better served by thinking about things that are real.