Persumably you want to get married because you want to share your life with somebody else. If you don't start sharing until you're 40, you've already missed the opportunity to share what are almost certainly the most transformative and significant years of your life. Of course this doesn't mean a later marriage can't be happy and successful too... but it does involve missing out on things that some people care a lot about.
"If a person wants to get married, s/he wants to share his/her life with someone" is (more or less) true, and is the premise everyone is working under.
The converse of that proposition is "If a person wants to share his/her life with someone, s/he wants to get married." That is what you for some reason appear to believe is the premise we are working under. But it's not.
The social structure accepts and enforces the marital union, providing significant legal protection to itself, the children, and the married partners.
This grants a solid staying power, borne of the subconscious knowledge that no party can really "just leave" and be done with it. This staying power should not be underestimated; it is effective and real regardless of how little one believes they will need to rely on it.
In the US/UK, you don't have to preemptively deny it, but if people begin to assume you're married and you don't correct them... then you're married under common-law.
I have a dear friend who has been 'dating' his girlfriend for almost twenty years. Their eldest child is 14, and they own a home together. As far as sharing their lives together, they've done it without being married legally---but in the eyes of all their friends and their kids? Oh those two are so married.