I don't think forums attract these people. I think forums make them more visible and impossible to ignore when they turn up.

The Internet has no quality filter, so as all posts get equal prominence - which means that if you don't shadow-ban them, toxic idiots, kooks, and those with personality issues get exactly as much screen space as people whose words you want to read.

Before the Internet these people would write letters to magazines, newspapers, and famous people, and their efforts would be binned, possibly after some laughter at their expense.

Internet systems give them a much wider audience.

More worryingly, politics has no quality filter either. Some of the extremist parties rely on these people for their membership.

It's hard to know how to deal with this - and it's potentially far more of a problem than forum trolling.