There are several points of view:
Already listed by OP where the value of the relationship has no direct link to the possession of marriage certificate.
Many young (and not so young) individuals are totally self-sustaining, and with an abundance of friends and earning/spending power, it's not at all pressing to get married and force to co-exist with another person just due to the historical precedents.
(One of my friends, a practicing attorney, was in a relationship for many years, until I just had to open my mouth and make a comment, "Well, it's another tax return prep time. Life's short. Might as well get married". She took it to heart, and in a week's time they were engaged, and married about a month ago. She will not be changing her last name).
Used to be a requirement (goes back to lack of voting and lack of property ownership privileges), complete with name change. Having had gone through just the first name change many years ago, I can say that it takes months for the changes to flow through (bank, credit cards, student records, passport, driver's license, rental agreement, work records, etc) - I knew that I just won't survive another change, so it was an easy decision.
Many females (and some males) are not into the proposition of working hard, so after the asset valuation is complete, they just sign up for stay-at-home marriage - the so called gold diggers.
Sometimes is makes sense to combine the fortunes to create something bigger - a strategic business decision. Other times require a prenup.
This one I reserved for my own opinion. Many times the knee-jerk reaction is to say, "Yes" fueled by peer pressure, parent pressure and other well-meaning external entities.
The only thing to say in defense of an intelligent, reasonable person is that marriage qualifies as a temporary lapse in judgement.