Yes, I do think it has something to do with a fierce desire to belong, esp. with the more 'family-minded' poly folks. I sort of resonate with what Deborah Anapol wrote about how single and newly-out polys feel they must first find their primary to start a family with before starting to add members. And how you can go around that with choosing a family/couple to join instead.
It is tempting, and yet oh so hard because the idea of chosen families is still so new and outside of the mainstream to many people. Like you said, there is this ceiling of 'this close is how you can get, this is what we are comfortable with right now' that seems to be in place for so many people.'
I sometimes think if it were better for single polys who are dating from that 'family/this is a lifestyle choice for me' place to find secure not-necessarily-romantic family arrangements first and then start reaching out romantically from a secure base/foundation of their own.
Me: bi female in my twenties