Because if you don't bother to look at why you're jealous, even in a monogamous relationship, you run the risk of ruining your relationship because you have no idea why certain things make you feel the way you do. You're stuck in a static position that doesn't change because you don't care to deal with your issues.
Poly or not, exploring your reactions to certain things makes you a better person and better able to articulate your problems rather than just saying "I'm jealous but I don't know why and I shouldn't have to know why!"
If you're having such a hard time with your husband going out and meeting people, why did you agree to have a poly relationship with him? If it's not something that you're able to do or deal with, wouldn't it be better to discuss it with him?
Also, I think that the comparison to family and friends and children is an apt one because it shows something very clearly: there are different kinds of love. Why can't there be different kinds of romantic love, as well? Is it that romantic love is a threat, whereas familial and friendly love is not? Friendly love can turn into romantic love.