Because, as far as we can tell, everything that is part of our universe is governed by physics.
Our brains are part of this universe, and conciousness is dependent on our brains.
Our brains are part of this universe, and conciousness is dependent on our brains.
This is neither a logical/mathematical proof, nor an empirically established fact. So how do you know? Why are you making statements of faith in what seems to be the scientific worldview, when the actual scientific worldview is that we should never make statements of faith?
Roughly 68% of the observable universe cannot be explained by physics ("dark matter").