As someone who has had more than my share of MMFs over the years and always enjoyed them, my view is that they're not more addictive per se. The thing with punting is that you can get to experience a lot more niche activities than you ever could in civvy life... So can stumble on things you didn't know you enjoyed, such as MMF. You can then repeat the experience as much as you want. So my guess is it's less a case of them being addictive than having repeat opportunities to indulge in things you enjoy!
Perhaps the more interesting question is why we MMF fans enjoy them so much. I agree with everything the OP said about why they are such damn horny fun. There's also the influence of porn. But of course there must be a deeper reason, bcs some men are horrified at the thought of being around another erect cock. Any psychologists, amateur or otherwise, want to explain the appeal of the MMF threesome?