Explain Like I'm Five

Yeah, like the other person also mentioned Counter Strike has had a major cheating problem for two decades and it's still pretty bad today. Valorant is a very similar type of game: twitch shooter that needs fine motor skills and reaction time where one player can dominate an entire match. Valorant has a more intrusive anti-cheat and a lower ratio of cheaters but both game still have cheaters and cheats. People will pay large monthly fees for access to premium, not-yet-detected cheats to compete in competitive circuits.

What's distinct about twitch shooters is that the core gameplay is very simple (just click on everyone's head) but it can take thousands of hours to become really competitive at them. People who are not at the same level as their opponent may think they are cheating if they outskill them enough which leads to a feedback loop where new players feel like they need to cheat to be on equal footing because the other person HAS to be doing it too.

Players with a lot of hours can usually tell if someone is cheating with relatively high accuracy (except at very high skill levels where the cheaters are also incredibly good at the game) but newer players tend to consistently call cheats on players that are just better at the game. Competitive drive, lack of trust in other players playing fair and high skill ceilings all create the demand for cheats which in turn creates lucrative opportunities for cheat developers.

Ruining other people's fun is also another popular reason like you said but I would say most cheaters justify it to themselves in some way.