It's not a mental block, it's because it is plausible. Just because it hasn't happened yet, doesn't mean it won't happen. In the extreme scenario, if technology advances to the point where it does nearly everything better than humans for practical necessities, it is unknown how much that will affect employment and society as we know it. In my mind, it's not a matter of if it will happen, but how far away we are from that reality. We could be 100 years away or 1000 years away and that's where the more interesting argument lies.