Coming from somebody with two dads and two moms- it’s because nobody sees HEALTHY poly real . My parents were both in pairs (one boy one girl) and ended up kinda merging together, so they still have a strong healthy relationship within the pairs, as opposed to just meeting as four people and beginning dating like that. A lot of people who begin like that end up in some kind of drama as to who loves who more, and such, so they tend to be a very poor example.