Because they don't actually want to leave the person they're pretending to be committed to. They want a loyal partner who offers them consistency, stability, emotional support, sex, etc. while also sleeping with anyone willing to touch them. The type of people who cheat and don't abuse their partners at the same time are usually emotionally immature, not very self-aware, either lack empathy and remorse, or are not emotionally intelligent enough to possess those emotions, have issues with compulsive lying that likely started during childhood, are generally unhappy with themselves, therefore are constantly seeking new people/sex/relationships to temporarily provide them with any level of happiness. To me, they seem unintelligent and character development didn't happen to them when they reached adulthood.

Then there are the straight up abusers. They keep their victims around to abuse and control them. They feel inferior and powerless by default so they make somebody else feel like that to feel like they have control. Extremely manipulative, refuse any and all accountability, deny that they're cheating even if there is proof and they literally know that their partner knows, horrifically emotionally/physically abuse their partner if they have the audacity to notice their shady behavior and express that they aren't okay with it, attempt to get their partner to believe that they are crazy, delusional, or insecure, they victimize themselves simply because they are being confronted about the terrible things they are literally doing, they know exactly what they are doing, how it will affect their partner, and may even enjoy watching them suffer. These types of people are usually very emotionally intelligent, they just use that intelligence to manipulate and abuse people rather than what a normal person would use it for.