Because the evidence strongly indicates so: we haven't found a single instance in the entire universe of a conscious entity that wasn't also a living being.
And 200 years ago only living being could fly, but now we have machines that can fly higher and faster than any bird, even to Mars. But back then you could have said flight requires a living being because that's all we've ever seen.
A hundred years ago computers didn't exist and now they've been beating us at chess for some time. That you haven't seen a conscious computer yet in no way proves that it's not possible is principle.
I never said that the evidence proves that conscious machines are impossible. I said it strongly suggests that it's impossible. Also I disagree that air-planes can fly, for the same reason that I don't agree that boats can swim. The term "can" implies agency, and machines don't have that. All the technological progress during the last 200 years doesn't appear to have brought machines any closer to having such capability.
