Because it does. Why do charged particles attract or repel? Why do some particles experience mass? At some point the answer is simply "because that's how the universe works."

We know consciousness exists, as we each have first hand evidence. If we want to believe that we live in a reducible universe, then there must be some reduction bringing consciousness down to a most basic physical process. At some point that reductive explanation ends with a very unsatisfying "because that's just how the universe works."

But I would be very suspicious of any model which reached that level before arriving at the level of fundamental particles and their interactions. Why? Because every other phenomenon in the universe also reduces down to that level, so why should we expect the explanation of consciousness to be different?