I think this works only after completing post school education & training and being in work for a couple of years, so say by 25.
You are an adult by then, and if you are looking for a long term relationship with, two years of dating is long enough to know if someone is right or not. Then a year or two planning the wedding, to see what they are like under pressure.
If you are a man, you know that having children is very financially risky for women, and puts their career back, so the protection of marriage means that what you have been building financially as a couple is recognized, and she isn't literally 'left holding the baby' if things go wrong.
People who have a level of financial literacy understand that if a shared house is bought together then if an unmarried partner dies, without the tax exemption of marriage, many are forced to sell in order to pay inheritance taxes.
People understand that a spiteful parent can refuse a partner visitation in hospital as the unmarried partner is not next of kin. There are no spouse benefits.
Some might have a philosophical objection to marriage, in which case they can make sure that they might that they manually ensure that their unmarried partner is next of kin in things like life insurance policies. They legally draft power of attorney documents they legally draft medical next of kin documents etc.
All the much more simple way to grant all these rights to your partner is to sign a marriage certificate.
Most people who do not get married do not bother to do any of this believing in some magical common-law marriage which upon things going wrong either death or separation they discovere they have no rights.
The majority who do not marry also do not bother to ensure there is any paperwork in place. They just want to make it easy to leave if they want to. Which is the opposite of the commitment marriage is.