They don't exist and there is not a good answer for "why". The idea of open or semi-open relationships in the US goes back to at least the "Free Love" movement in the 60s/70s and nobody remembers the beautiful love stories from that because they don't exist.
My personal theory (for which I really wish I still had my Hannibal Lecter avatar) lines up pretty well with yours, it's a way for people to pick and choose the parts of a relationship they like because they can't handle being responsible for another human being: I know two or three people IRL who self identify as poly and it's pretty clear they are trying to have all the romance and dates with none of the disagreement, fighting, and so on. Even when their "polycule" (I hate that term, it's polygamy by another name) had no choice but to admit things were going poorly they couched it in Disney references. This is not a joke and it took a lot of restraint for me to not say "what is wrong with you people?".