Why do Cheaters Think They're so Privileged?

I recently started getting invested in revenge style reincarnation novels and while I was reading the novel "Reborn To Love," I kept finding myself get annoyed with some of the characters. (Mc's stepsis and her ex-boyfriend)

Before that, I read other novels/manhwas where the antagonist (also known as either a stepsis, half-sis, adopted sis or even a bio sis) would steal the mc's boyfriend away from her. Everytime I read these things, it makes me feel so infatuated, 'cause why would I want to read from a cheater's point of view who'd unecessarly hurt their "sibling" because they don't like them?!

But that's what authors do right? That thought process led me to think, "huh, real human beings think like that too." It's honestly so disgusting to me that people think that cheatings okay. Like, how are you going to destroy a relationship for the fun of it, only to end up with someone else who will probably end up cheating on you too?

I will never understand why "cheaters" (Both the person in the relationship and the person who's the "other" person; they know they are) think all of a sudden, "oh, I got him/her, I don't need to worry about them cheating on me." NO, you were dating a cheater, they're not going to stop for you and they prob will stop never.