People don't want to marry for a range of reasons OP.
Some might have had bad experiences of marriage and not want to do it again
Some don't want to be legally tied to another person
Some might have children from previous relationships and marrying a new partner means their children could very easily be disinherited
Some don't believe in marriage as an institution and have structured their affairs in a way that works for the couple
Some are happy cohabiting and remaining financially separate
Some might want to remain legally single until one partner has resolved some financial issues
Some might not have lots themselves, but are aware of their family assets and don't want them to be pooled
Some might be clued up on the legal and financial implications of marriage and don't want to sign up to them
Some might be concerned that if there is a large difference in income/assets and their partner has no interest in working or remaining financially independent that they'll lose a hefty chunk of their assets and pension in a divorce
Some are in relationships that they see as 'right now' relationships but if they end up single in future and meet someone they want to marry then they'll marry quite quickly
In a nutshell, all adults should get clued up about the legal status of marriage and make an informed decision regarding marriage and their financial situation.
Too many women don't get clued up, have babies with men without thinking about their situation, give up their financial independence and then later if it goes wrong complain that they should automatically have a claim on assets because they live together and have a child (aka because they they didn't make an informed choice, they think other women should lose the right to cohabit with a partner and remakn legally separate).