Pracaticality.
If you are close lifetime buddies you might get less static if you want to live in the same house.
But what I experienced with my close platonic friend was when he was in the hospital dying it would have been much better had we been married so I could have been there more and in on decisions. I still was but it was a major hassle and when the marriage solution dawned on me he was too ill and confused to really be competent. This hospital thing is also a reason many gays give for wanting to be able to marry.
I can see where that plan would be an advantage. I still think that our society needs something other than marriage that would give the same advantages without specifically being a marriage. I see marriage as something that is supposed to have some very specific meaning, and if that meaning doesn’t exist, I hate to see marriage used that way.
Good point about being ill and hospital I think hospitals are more understanding about couples not married nowadays especially when younger and they have young children but I can see when older they do not accept couples together so much who are not married.