For the exact same reason that women accuse men of cheating when they’re not: latent insecurity and possible self-sabotage.
Some people don’t trust anybody. And despite trying to engage in relationships, they are always waiting for the other shoe to drop. So the second they see ANYTHING that could even remotely resemble inappropriateness, they jump to the furthest conclusion (cheating) that they can come to. That way they get you before you get them.
People with trust issue are always out to make sure that they never get played for the fool. The problem is, they often shoot first and ask questions last in attempts to stay ahead of the game. Which, obviously causes all types of friction within a relationship because why would anybody want to be with anybody they can’t trust.
Though for some odd reason, people try. And fail. But they do try.