In the very few places where common law marriage is allowed, it still requires the couple to hold themselves out in public as married. It does not "trick" people into getting married if they don't treat themselves as married.
There can be complications if one party claims to be common-law married and the other does not, especially during divorces.
Those factors can be, amongst others: joint financial accounts, mortgage, title to assets, children, vehicle insurance policies, reciprocal wills, reciprocal beneficiaries of life insurance, health coverage, intimacy, etc.
What I’m getting at is that it’s not that hard to hold yourself out as married when you’ve got so much “entanglement.”
That’s not always the case. Here in Ontario there was a case of a relationship being declared common law by a court despite not cohabitating. That is quite unusual, but simply living together for 3 years (or 1 year with a child) is the normal yardstick courts use.
There can be complications if one party claims to be common-law married and the other does not, especially during divorces.