I think it is primarily because there was incentive to do so. To be clear, I do not think it was faked. However, I think the conspiracy is more believable given that we were in a race with the Soviet Union during the cold war to get there.

People feel invigorated when baseless conspiracy becomes legitimized by politics. Same logic that drives "masks/vaccines don't work" mentality.

Contrarianism. They want to be above other people, aren't above their peers in terms of knowledge and then create a narrative where they are above their peers in terms of knowledge.

Disclaimer: I have very little faith in the majority of conspiracy theories. Most are stupid and the people believing in them are even stupider.

These stupid people provide perfect ammunition for malicious governments though.

The advantage of having "stupid" conspiracy theories prevail (such as this one) is that some of the other conspiracy theories that are rooted in some truth can be brushed off as mere conspiracy theories. Governments have little incentive to prove beyond doubt that the moon landing was real as they can use this to hide other things they actually covered up.

Because it was.

They didn’t DYOR and have a thin grasp of the science involved and resulting tech that infiltrated our lives.

Also, we haven’t been back to see the landing site.

Soon ™️

Obviously, there are a bunch of different answers to this question and, as someone who thinks the moon landing did happen, I'm not exactly the right person to answer this. However, two of the oddities that make people suspicious are that the original footage was deleted and one of the space walk photos was clearly an edited training picture that was taken in a pool.

We discussed this in the recent conspiracy theory post. My view is that some (maybe all) of the promotional material was faked. That might have been because they realized (or worried) that radiation (or some other technical issue) would mess up real pictures and video, but they knew the public demand for documentation was enormous.