Because sex = validation. "I'm so amazing! Look at how people want me and tell me how amazing I am!"
I suppose it could be, but honestly, I think it's so much simpler than that.
Why order dessert after a big meal when you're already full? Because you can, and because it tastes good. In a lot of ways, it's really that simple. I've never cheated, but I've done a lot of dating in my day, including dating multiple women at the same time? Why? Because sex feels good, it's fun, and when you have more women in your life you have more opportunity to have that fun and different kinds of fun with different women.
The hurdle for me isn't having more sex when you're already having sex, it's risking so much for more sex. And, I can see why, it's just not a great cost/benefit analysis, affairs don't usually last that long, it's a lot of work, and they often end badly, including going on a sexual drought with your H/W. So, in effect, your borrowing tomorrow's sex for today. For a lot of people, myself included, sex with someone new is usually quite good, so, you could argue your trading maybe not great sex for great sex but.. Honestly? I think it's just a failure to see into the future. Same reason people borrow up to their eyeballs in debt buying crap, they can't see into the future and how badly all the debt will hurt them and come to define the lives. Many, many people suck are it; they can't see to the end of the day, let alone months/years from now. Hence, retirement crisis, people whooping it up and partying while there's a pandemic raging and, of course, people risking a good marriage/sex life for a quick toss with someone new.