There’s a difference between denying that (1) we can’t know (2) that we’re not dreaming, and denying that (1) we do not in fact know (2) whether highly speculative empirical claims about very complex systems (societies, human history) are true. Objectivists routinely argue that rejection of highly speculative empirical claims is tantamount to rejecting the possibility of knowing anything. Cartesian skepticism, in turn, is associated with rejection of science and technology, moral indifference and the condoning of evil. Peer pressure and vanity does the rest. This way one can come to think that one is morally obligated to believe things for which there isn’t sufficient evidence, and moralize against those who disagree. Much of the psychology of this is common as grass, but the use of Cartesian skepticism as a strawman is, as far as I know, unique.