I suspect it's due to the single largest obstacle we encounter when attempting to build fulfilling relationships is ourselves. It is our struggle with ourselves that determines how well we are able to treat others well and assist them in building relationships that are fulfilling to everybody involved.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.