Even well intentioned people go from “visibility” to the edge of the “no visibility” and due to the fog can’t judge that there are stopped cars just within the “no visibility”. So they drive into the “no visibility” try to stop, and it’s too late. Thus the pileup has started, and as the fog expands/moves, it continues.
Yes, people should be ready to stop at any moment, but under certain conditions on the highway you can believe you are commuting normally then be compromised quickly. A bit of bad luck and when you realized you are compromised you’ve already hit someone.