Humans conspire _all the time_; the whole of human history is marked by innumerable conspiracies between humans to achieve shared goals.

Believing that it's possible for humans to conspire isn't wrong, it's rational.

Making claims without having evidence for it, believing that shadowy cabals of people are somehow running the world or entire organisations (actually pretty much impossible in modern times), adopting scientific language but actually not understanding how science works (anti-vaxxers, 9/11 truthers, flat earthers etc..), making extraordinary claims or rationalisations without having an extraordinary good reason to make them, generally being paranoid and framing everything as some sort of power play, believing that everyone who disagrees with them is part of some establishment or has a hidden agenda, and so on.

>Believing that it's possible for humans to conspire isn't wrong, it's rational

Conspiracy theorists don't just believe that it's possible for humans to conspire, they believe that humans conspire all the time and use it as an explanation for claims that don't hold any water. Virtually 99.9% of conspiracy theories are wrong even if one or two happen to be right by accident, so it's useless as a way of approaching the world.