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.
"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.