I’ve known quite a few people/groups that are poly and I dated someone who was poly for a while too. I did it because I didn’t feel like I had to deal with 100% of my partner because that would have crushed me.
My info is purely anecdotal but two super common threads that kept on appearing is there were people who were poly, but were never actually poly and just said it because their partner wanted to be so they said they were too and that the people who were super committed to poly all were trying to fill a gap in their lives and had a lot of insecurities in general.
Most hated the idea of ever being alone, not just in ‘a relationship’ but actually just being by themselves.