It is because computation is an interpretation imposed on a physical system from the outside, but that is distinct from my experience of consciousness; your interpretation of my consciousness is irrelevant.
If I impose a computational interpretation onto inert material after the fact, did that material possess the consciousness? Suppose I overlay a projection of the calculation of an AGI onto a circuit board, is that projection equally conscious to if that circuit board did the computation in the usual way?
Said differently, suppose we take a large computer and simulate a sequence of random byte strings. At each tick we contrive a substring and form a new sequence out of these substrings such that the combined sequence of substrings simulates an AGI. Was consciousness present in the original sequence. Is it even necessary to do any computation at all to create the consciousness then since any intelligent sequence can be interpreted out of anything?
No, that is absurd. The reasonable view is that computation is irrelevant and what is relevant is some special physical process.
You're explaining the part I said I understood instead of the part I asked about. I don't see any reason computation would ever have to lead to true subjective consciousness instead of an externally identical philosophical zombie.
But the alternative is equally absurd. In essence, you have a bunch protons, neutrons and electrons pushing and pulling on each other and somehow that causes consciousness. There just aren't that many weird properties or particles to appeal to as a cause of consciousness. While there are exotic particles and the quarks and gluons that make up protons/neutrons, these particles are relevant for our study of the universe, but they aren't relevant to understanding the human brain. Abstracting protons as a single particle that push and pull on other particles via forces works fine for explaining everything we know about the brain.
Not that it really changes anything if the particular way quarks interact did affect the brain in a way that couldn't be explained through the simplified view of a proton. It adds a few more particles to consider and the weirdness of quantum chromodynamics, but nothing there explains consciousness either.
So how do you go from particles pushing and pulling on each other to consciousness? It seems to me no matter how you arrange a bunch of particles, there is never any reason to assume that arrangement is conscious. It's just a bunch of points moving according to a few simple rules.
If you're saying there is something more in physics, some other force or particle or currently unknown way particles interact that causes consciousness, that's roughly the soul particle idea.
Saying consciousness is caused by a physical process is very vague. What kind of process? Why don't the physical processes the computers running an LLM perform count? Why does the brain qualify? If we go on to build many different types of artificial brains (using silicon or biological cells or anything else), how would you even go about recognizing which artificial thinking machines have the required physical process for consciousness?