men at the top tiers of attractiveness don't even need to join the poly community to have multiple partners.

As I understand it, polyamory is having multiple committed relationships, not just multiple partners. I don't think having multiple partners is limited to only the "top tiers of attractiveness" for men; I think it's fairly common.

I doubt anyone joins the poly community just to have multiple partners; it seems like way too much work building relationships for that, when you could just find friends with benefits. Someone who just wanted multiple partners would likely not bother; however, people who have been in successful poly relationships likely have a lot of experience and practice in dealing with sex and emotions and managing relationships, which would increase their attractiveness.