You’re ignoring the pragmatic. You’re asking why they aren’t married when you may want to ask why should they get married instead. Why should people who are just getting by worry about getting into a legal arrangement that more or less just pre authorizes a guy in a long black dress to redistribute their assets after a bad enough argument?
Here in CA, if you stay in the committed relationship (as in, renting or buying a house together, having children, several years together) for several years, the court WILL have a say in redistributing your assets anyway if the two of you break up. So not getting married is not an advantage unless you take steps to make sure it doesn’t appear to be a committed relationship.