It is only a problem in physicalist view of reality. In idealistic, panpsychist or nondual perspective, it is the reality. In physicalism, consciousness is a problem because it is the property of being "about" something. Unlike a gear in a machine, it is something that is the aboutness of the machine. It is the screen on which colors appear, audio appears, self reference appears. Physically, optical and auditory signals are the same thing, just a different pattern of firing in the brain, but in sensation, one feels categorically different from the other. The fact that blue appears "blue", and that there is no definition for blue in a physical description of brain other than its neural correlates makes the problem more pressing. The other factor to note is the self referncing element which is essentially unavoidable with the experience. Each experience is accompanied by a self, otherwise it wouldn't be an experience but just an impact on the human. The problem is again, why do we have to use words like "experience" and "feeling" and "understanding" to talk about consciousness if it's just another phenonmena. What does understanding mean if it is just a retraining of a person's neurology? Why does it feel from the inside like "aha, I get this now". The very fact that we have to use words like "feel" to describe a facet of physical reality. Philosophically, you can doubt the existence of the entire outer world but not the experience of what enables you to do so. Hence, it is a clue that it might be more fundamental than anything else.
