I get what you are driving at, but if you live in a culture that doesn’t glorify ego and separation, it’s mostly not going to come up. The Na of China kind of look down on jealousy and even other current cultures that are more centered on pair bonding frown on it. Enforced monogamy is a patriarchal structure. We didn’t have it before that social system came into being. If someone wants to choose monogamy that’s fine, but most people don’t acknowledge that they have any other options and it’s so overwhelmingly supported in society that for all intents and purposes, it’s still fairly enforced.
Patriarchy is a social system based in a dominance hierarchy, which means that enforced monogamy is too. When humans lives more non-monogamously, they lived more cooperatively in other ways as well. So I see those two things as very much going together. In other words, you’re right about culture, but it’s the cultures that demand monogamy that are the most jealous and possessive.
What I’ve most gotten out of polyamory is an understanding of myself as an individual who co-creates different kinds of relationships with different people. I don’t own them and they don’t own me — not even my primary partner and that’s been very empowering and made those relationships better and stronger. Sometimes poly people experience jealousy but I think that’s mostly from living in the heart of a dominance hierarchy and trying to deprogram yourself from that.