(NSFW) Why are some people not interested in a monogamous relationship?

I know it can be exciting for some playing the field (if you will). But how long can that excitement really last? Personally if I was to just to have casual sexual encounters; quite frankly I would get very lonely very quickly. I would find it meaningless and empty.

In my opinion, when sex is used as an expression of love within a relationship; it in my view enhances the experience.

So I guess I’m trying to understand why someone would not want a relationship.