Why are long-term couples reluctant to marry?
Marriage offers a lot of financial advantages over remaining single. Since "no fault" divorce is the law [nearly?] everywhere, untangling the relationship isn't so bad even if you do call it quits. Actually, because it's a settled legal process, divorce is probably easier than sorting out an extra-legal arrangement. So why the reluctance?
Common law marriage is recognized in only a few states and one of its requirements is that the couple tell people they are married, so you don't get CL married simply by keeping house together. Divorce law gives each partner power to force a settlement - the court can impose one. Without marriage, one partner could stall forever. And a spouse has rights when their partner dies or becomes disabled, sick, &c. If you die intestate an unmarried partner gets zilch if their family steps in.