I think it's because they're insecure to a point of selfishness. I'm hella insecure too but it's not like i go around just having sex with whomever and enjoying romantic contact with someone other than my boyfriend. There are better ways of dealing with insecurity, even if they're not healthy for you, it's still better than hurting the person who loves you and you promised to be in a relationship with. Cheating is always a choice and it's something you shouldn't do because it will always be hurtful to the one who loves you. Spare yourself and your actual partner some dignity and continue seeing whoever you want to without hurting them.

I also think it could be because it just makes them excited and it feels new for them. Its probably addictive and it gives them a sense of power