Me and Ad are crying into our cockles and laverbread right now.
I don't like to think of people as being "just" stupid. I think it's more interesting and helpful to think about why they believe what they do.
There was a big push in the media about how "now it's all over we need to go back to how things were". I think a substantial chunk of people realised from the experience of the pandemic that, actually, wearing a mask is a decent choice in winter just to generally get less ill -- but they were portrayed as childish, moralistic worriers, or even basically monsters who want us to be locked away forever so we can never hug our parents and the kids can never play outside again, and so on.
Not everyone has the critical thinking skills to read all that and realise that it's propaganda from a ruling class who are invested in maintaining city-centre commercial building prices and generating culture wars. Unfortunately, many people will take it at face value -- especially as it wasn't just ridiculous tabloids taking that line, but also "respectable" sources.
That doesn't excuse the bullying behaviour when it arises of course, it's totally unacceptable. But the propagandists and bullies have largely won, it's quite difficult to bear the social consequences of wearing a mask. I only mask up on long train journeys these days and even then it's a bit embarrassing getting so many weird looks.