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People have always believed in outlandish conspiracies. For example The Protocols of the Elders of Zion was debunked over 100 years ago but still went on to influence the Nazis after that as well as countless other antisemitic groups and conspiracy theories to this day.

https://encyclopedia.ushmm.org/index.php/content/en/article/protocols-of-the-elders-of-zion

Convincing people of something they don't believe with evidence is almost impossible, as anyone who's spent time on the internet knows.

So how are these conspiracy groups convincing people without evidence? Simple, they're not. They're providing validation for the ideas and general misgivings that certain people already had, as fringe groups have done for generations. It's just that in the past it happened privately, now it's taking place in public forums.

Maybe the internet allows dumb ideas to spread faster. But it also lets criticism of those ideas and evidence against them spread at the same speed. The problem is the people who need to hear it will never listen.

TL:DR - Some people have always been stupid and looked for validation of their beliefs, now they do it online instead of IRL as with everything else in life.