So my question is, does the philosophical zombies question/idea contain a hidden assumption and why (if/because it does) is this assumption (that normal humans are conscious and have qualia) not explicitly stated when the zombie question is asked?

Yes. All questions do. And all answers. And all arguments. This was noted and explicated on by Quine with his notion of confirmation (epistemological) holism in his exploration of theory ladenness which has a perceptual and semantic aspect. While the discussion is had among philosophers of science, the notion that there is a holistic aspect to epistemological thinking permeates contemporary philosophy of language with its focus on expressiveness, productiveness, and compositionality. Semantic epistemological holism is the thesis in its moderate form that propositions are only meaningful within the context of the situation. Thus, there is the larger body of text (endophora), by examining references outside of language (deixis), refering to meaning not specifically encoded (implicature), and paying attention to the intentions of language (Sprachspiel). This is what Quine may have been signalling when discussing the notions of web of belief. Every argument made has hidden assumptions, because no one spells out one's worldview before crafting language. The question becomes is it necessary to include the assumption? The one you point to doesn't seem necessary since it is almost universally presumed to be true.

So, is it unstated that human beings have consciousness and experience qualia. Yes. Is this surprising? No. The objective of the argument is to challenge that intuition and force the thinker to deal with the notion that other human beings might not have minds. This isn't isn't revolutionary. It's solipsism in a modern language.

In Ancient Greece, three important epistemological problems unfolded themselves. The first is the question of the existence of knowledge. Why shouldn't we be radical skeptics? The second is the question of the difference between appearance and actuality. Should we accept naive realism? And the third is how do we know other minds exist if we only can introspect our own? This is the problem of solipsism. Needless to say, most modern thinkers move beyond Pyrrhonism, naive realism, and solipsism and accept knowledge exists, accept representational theories of mind, and accept that we may infer other minds exist. Thus, the problem of the philosophical zombie is an attempt to push back on the last two to accentuate that there isn't consensus on mind-body duality, which is another problem that seems to arise.

Why do people hide the assumption contained in the philosophical zombies question/idea?

So, the assumption is not hidden. It's just so obvious it isn't mentioned in these sorts of conversation, because the notion that humans have consciousness is powerfully intuitive. According to a naturalistic approach to epistemology, the reason for that is straight forward: human beings are genetically enabled to generate theories of mind and to use collective intentionality. Like breathing and using our opposable thumb, it's simply natural to understand and collaborate with other people; in fact, it is thought by some researchers that autism is a condition that is caused in part by neurodiversity leading to mind-blindness. I suspect most psychiatrists would consider radical solipsism, an unfettered inability to believe in other minds, a presentation of a symptom of a mental disorder. People who sincerely believe they are Jesus, or Elvis, or aliens, or the only person really alive behave in ways that confirm it, and lead to social situations that support therapy and institutionalization in the extreme.

The sort of people who engage in vibrant debate about mind-body duality, reducing the mental to the physical, and can discuss intelligently eliminative materialism and subjective idealism presumably all have the intuition that other people have conscious experience. Thus it's not usually explicitly spelled out for parsimony.