Atheists claim not to believe in God, but what they actually deny is the interconnectedness of all things. Religious people, in believing in God, believe in the interconnectedness of all things through God (God being infinite, and thus disconnected from nothing).
So when Atheists claim that they are “rational” or “reasonable,” inevitably they mean these terms as applied to the “reality” of fundamental isolation - so, to someone who is completely isolated, “community” becomes the overwhelming goal. Thus they see compelled collectivism as the ultimate solution - whether in the form of socialism, communism, progressivism, totalitarianism, or any other mandatory group organization.
This is why liberals not only disagree with, but are so frightened by conservatives - conservatives fight for individual freedom, which liberals see as forcing them to be alone in an isolated universe. It terrifies them, so they fight with the fury of the damned against it, because that’s exactly how they feel.
Except in the case of conservative atheists.