Well, only 9 states recognize common law marriage:
Common Law Marriage
You posit common law marriage (and common law divorce) as a simple thing, but when assets become involved, and when alimony claims float to the surface, it's a real pain in the ass to sort through everyone's claims.
See for example:
Jennings v. Hurt
Common Law Marriage Article. New York Divorce and Family Law, the definitive site about divorce, child support and custody.