I find that it is a sign of those who need and deserve to be kept in their own communities and not to socialize much with others. Stupid requires some restraint. As for cause, It seems more to be a thing of information overload of the society, so too much info, go for the overly simple if not proven wrong.

What's wrong with anti-maskers and anti-vaxxers?

The people representing them or the people themselves? My father had a few pretty strange beliefs that many would have classified as conspiracy theories. I think that believing in these sorts of ideas isn't so dangerous as the refusal to engage in fair and meaningful debate about certain topics. Should people wear masks during the pandemic? I believe so, generally - but I also believe that there are times and places where it makes no sense to wear them and times and places that are ambiguous as to whether they provide meaningful protection and times when they are absolutely foolish not to wear. Same with vaccines- there are cases where a vaccine is totally advisable and cases where injecting yourself with something that someone is calling a vaccine could be extremely bad for your health, and, of course, there are cases in which it's unclear. The flat-earth thing is maybe the only one that I'd say is 100% settled, but I still believe that it's important for us to have challenges to our mainstream ideas, because it is important to understand why we believe the things that we believe and not just to believe them because our parents/teachers/government/temples/churches/royalty/authority/etc. told us so.

Just because people disagree with you doesn't mean they are wrong. Some people have weakened immune systems or allergies that might make the vaccine unsafe. Also they have not been proven. As you see with the blood clots in women of child bearing age. Anti maskers might be asthmatics or just have one lung, or on metoprolol which doesn't allow for your heart to speed up during exercise to get the oxygen to the muscles. These things make it difficult to breathe. And all I can say about flat earther's is that they are probably victims of internet programming that feeds them what they want to hear and they don't research the opposite opinion to balance that mindset. Its just like people with political bias who never open mindedly listen to the other side, or look at it from the other position. I've been a victim of the same thing, firmly believing I had accurate information and couldn't believe anyone would disagree until I looked things up from the other side.