I agree with you, but we're also poly (really, mono now but still capable of poly), and we have experience setting aside the jealousy impulse.

Imagine this: Imagine a society where, not only are all people generally angry, but some are viciously wrathful, and people regard it is normal. Except, really, it's worse than that. If you're not generally angry all the time, people treat you like something is wrong with you. You're expected and pressured to be angry, and if you don't tolerate others' anger to the point of abuse, then people regard you as wrong for it.

Similar analogy: Imagine a society where being clinically depressed is the norm to such an extent that even psychologists consider it a given that everyone is clinically depressed. Evolutionary psychologists speculate about how clinical depression raises the probability of successful reproduction. Family court cases are presided over by judges who evaluate how clinically depressed people are to decide cases, and it's a strike against you if you don't make every decision under the assumption that your partner is clinically depressed.

Jealousy. Anger. Depression. These are in the same category of emotional and psychological states of being, but one of them is regarded by society as healthy, normal, understandable, unavoidable, and is encouraged outright. It's kind of sick when you realize it, but can you blame people for failing to realize it when jealousy is the one selfish, impulsive emotion carried over from childhood for which nobody is usually pressured to develop mature self-control and rationality?

Speaking in general terms, of course. There are pockets of healthier people here and there.

Okay, so suppose you were talking to someone from the clinically depressed or angry society. You can't come to them from the perspective of somebody who understands the cognitive skills involved in combating or mitigating the negative effects of those things. You have to keep in mind that they've been trained their entire life that they're expected to be angry or depressed, and anyone who isn't is sick in some way.

Starting there, you'd have to acknowledge the prevailing thoughts driving their emotional state. In our society, fear of loss drives jealousy to the point that nobody is really expected to have basic self control where it's concerned, unless they go to extraordinary extremes. And even then, people excuse their behavior. "That's just how they are. Those are their values. That's their culture."

So, I acknowledge the form their thoughts take. They fear losing their significant other. Really though, that's not what they fear. The real fear is being alone, and fear of going through the process of having their brain regard a separation similarly to physical pain. Instead of being told to take ibuprofen to help manage the pain, people are treated like the pain is a weakness (even though it's involuntary), and jealousy is regarded as understandable.

See why I put it that way? The people I'm talking to aren't where you and I are. They may never get there, after living their lives to this point immersed in this culture. I don't hope to talk them out of being jealous, and I completely understand that they may not even be able to achieve that. All I hope for is that they'll soberly evaluate whether their jealousy is justified, treat their significant other well, and avoid staying in a relationship that hurts them.

If people do that much, then all is well. Ultimately, we just don't want to see people suffer, right?