We as a species may lack the mental faculties to truly understand the mechanisms by which life evolved the ability to analyze and reason about its surroundings and itself.
We lack the scientific tools to properly differentiate or falsify "consciousness" from any other sufficiently complex phenomenon. We can't tell whether it's an emergent phenomenon arising out of a particular arrangement of energy and matter, or whether it requires some input that we cannot currently observe/measure, or something else entirely. We don't know whether we can create consciousness from its constituent parts, or if it even has constituent parts. We don't know if there's a finite supply of it in the universe, we don't know if it's a dimensional thing, we don't know how it interacts with other forces.
We do know that it appears as though we have much more agency than a rock, yes, but it's a matter of degree how much more when we compare ourselves to other lifeforms, primates, dolphins, elephants, ravens... or other people. We don't know if certain members of our species are "more" or "less" conscious. We don't fully know what happens to consciousness during comas or brain death or dream states or sleep.
It's just an ambiguous term that we apply to the "state of human information processing that we can't really explain". Substitute "ambiguous" for "illusion" if you prefer, but it could also very well be an illusion the same way centrifugal force is a pseudo-force, i.e. the measurement of consciousness depends on some reference frame that we don't know how to use yet.
It's entirely possible that consciousness is NOT an illusion, that it is indeed a special "thing" in the universe, but we can't prove that with the science, language, philosophy, and possibly mental capacity that we currently have. Maybe one day we will. Maybe not. But it's premature to assume we understand anything about consciousness in the philosophical sense.