As others have already pointed out, that depends very much on the nature of the conspiracy. Like 9/11 was undoubtedly a conspiracy... by al Qaeda. Seriously it if you take the Wiki definition of a conspiracy:
A conspiracy, also known as a plot, is a secret plan or agreement between people (called conspirers or conspirators) for an unlawful or harmful purpose, such as murder or treason, especially with political motivation, while keeping their agreement secret from the public or from other people affected by it.
Then what they did was a conspiracy. So conspiracies do exist and yeah if something exists it is likely that it will be taken into consideration when it comes to the assessment of reasons for an event.
The thing is "regular conspiracies" are complicated and have a lot of dynamics that are directly opposed to each other:
- doing something significant -> draws attention and makes secrecy hard/impossible
- doing something insignificant -> still requires massive resources with little reward -> potential ruin in the long run
- being in positions of power -> hard to achieve and the opposite of secrecy
- not being in position of power -> Lots of more powerful enemies
- doing something major -> lots of people required
- lots of people = lots of points of failure
- To be effective insider knowledge is required -> Having insider knowledge means you're on the (short-)list of suspects
- Long planning is required to hide one's trail or not leave trace in the first place -> long planning means lots of chances to fuck up or have things change in the meantime.
- Short plans and action -> lots of things have to be estimated
- "absolute" secrecy requires to be paranoid -> going insane is not helpful
- And ultimately time works against you as co-conspirators write memoirs, cold cases will be subjected to novel technology that you couldn't have prepared for and you'll inevitably have left a lot of evidence one way or another. And hastily concealing them probably leaves even more evidence.
Though when it comes to what are colloquially called "conspiracy theories" you kinda get into the domain that is almost religious. In the sense that you believe in an entity that is all powerful, all knowing and almighty (* within limits) and that constantly, successfully interferes with the world on both a global and personal level without leaving any trace or being uncovered.
Seriously that could literally serve as a definition of a god.
And that's not even an exaggeration given that some literally believe in aliens, where the "advanced tech = magic"-formula comes into play or stuff like the simulation theory where the creator of the simulation would be some kind of "god" for that universe.
It's also a self-fulfilling prophecy in that if you believe in their absolute power to conceal their actions perfectly, then the absence of proof is proof itself.
And I'm not speaking of actual evidence like a used car being perfectly clean as proof of cleaning, but the absence of any evidence being evidence itself.
And the more your paranoia takes hold of you and the more you distrust others, the more it becomes impossible for you to verify or falsify any claim as you rely on the expertise and witness of others. Which you can conveniently dismiss as liars in on the conspiracy. Upon which you'd be approaching absolute skepticism up to nihilism and the inability for any sort of epistemology outside of the "fact" that the conspiracy exists.
So sure if you believe in a god that is able to interfere everywhere and at every time and level then that is THE primary variable of your universe that you have to account for and you'll potentially see that everywhere.
If you don't, then you'd realize that for any human actor or actors faking the moon landing is probably more difficult than going to the moon. Like you'd employ 400,000 people in the Apollo project that you'd have to provide with secrecy and/or bullshit jobs that don't talk to their friends and families about their job, that create conditions on earth that aren't possible on earth, that stand the test of time even up until now, that they were confident enough to show to 600,000,000 people, that have since been replicated or confirmed by other nations who at the time had no interest of granting the U.S. that success if they could do that. And the longer it goes the more extraordinary the secrecy measures would need to be, while the more insignificant the achievement would be. Like sure it's a historic feat, but if it didn't happen in 1969 but does happen now what's the point of hiding that knowledge in the first place.
So at some point you'd probably take a leap of fate into the abyss and if you do it's hard to recover.