There are those who called the Nazis out early on, even in the US. What I think you're missing is that we pile on when it's clear the tide is moving. In that case, it's about virtue signaling. Shaming, or really any behavior has risks and rewards. It's only when the rewards vastly outweigh the risks that most people will do anything at all. Humans are risk-averse. We wait until we get told we're dying to fix our weight and exercise, we wait until last minute to study for the test, we spend money we don't have instead of saving for tomorrow because who knows what will happen. So when people start piling on the shaming, others pile on even more, lest they are the subject of the shaming themselves.