First, many health experts, including the surgeon general of the United States, told the public simultaneously that masks weren’t necessary for protecting the general public and that health care workers needed the dwindling supply. This contradiction confuses an ordinary listener. How do these masks magically protect the wearers only and only if they work in a particular field?
Second, there were attempts to bolster the first message, that ordinary people didn’t need masks, by telling people that masks, especially medical-grade respirator masks (such as the N95 masks), needed proper fitting and that ordinary people without such fitting wouldn’t benefit. This message was also deeply counterproductive. Many people also wash their hands wrong, but we don’t respond to that by telling them not to bother. Instead, we provide instructions; we post signs in bathrooms; we help people sing songs that time their hand-washing. Telling people they can’t possibly figure out how to wear a mask properly isn’t a winning message. Besides, when you tell people that something works only if done right, they think they will be the person who does it right, even if everyone else doesn’t.
Third, of course masks work — maybe not perfectly and not all to the same degree, but they provide some protection. Their use has always been advised as part of the standard response to being around infected people, especially for people who may be vulnerable.