As much as I hate it, the biggest problem with reflexively dismissing conspiracy theories is that people in power conspire all the time.
If you'd told everyone walking out of the 1991 Oliver Stone film that "in 2017, President Donald Trump will push back the deadline for releasing documents about President Kennedy's assassination, and in 2021, President Joe Biden will do the same", anyone whose brains didn't flat out explode would agree that this was obviously nonsense. But here we are.
@mhoye Conspiracies certainly happen, but they are most often of the more banal variety. To me it seems like many popular conspiracy theories fail on other grounds, like
1. they require extraordinary competence of the supposed conspirators far beyond what is in evidence,
2. they don't really provide adequate motivation to justify the potential costs for the various conspirators, and
3. they overestimate how easy it is to keep a secret when many people are involved.
Well, there's also the more mundane reason, which is that they endeavor to explain something that has a much simpler and more straightforward explanation (but which is less interesting and emotionally satisfying).
