Because we know what it isn't. It isn't just processing more data.
But what
is it? And why can whatever-it-is
not be implemented on a non-biological substrate?
If you want to believe an AI program is conscious, be my guest.
I don't believe that any AI program currently running is conscious. I just don't see any reason why a non-biological system could not some day be conscious. I don't see anything about consciousness that makes it
inherently impossible to implement on a sufficiently complex non-biological system, in principle.
In fact, I'd go further, and suggest that in order to make it impossible for consciousness to exist on a non-biological substrate, there must be something
special about biology that differentiates it from non-biological systems - something
other than mere complexity. Dare I say, something immaterial and nonphysical. Because we (ie., science) understand the physical matter pretty well.
We cannot currently simulate an entire brain on a computer, because a brain is the most complex structure in the universe and there is no computer yet powerful enough to do so in real time. But we understand quite a lot about
how the component parts of the brain - neurons, particularly - work. There's nothing about them that couldn't, in principle, be simulated on a sufficiently powerful computer. Do that, then link trillions of them together into recursive feedback loops, give it inputs analogous to senses, and outputs analogous to communication, and why shouldn't it be sentient? Why
couldn't it be?
The only reason it
couldn't be is for there to be something that consciousness has that is immaterial, nonphysical. And I don't believe that souls exist.