Because their dating culture kind of demands it, and there was a time where women couldn’t have a bank account without a man signing off, and of course people were getting married and having children super young (my grandmother was like 18/19) so it kind of made things difficult to break up when you have a toddler to raise, and the final nail in the coffin was no fault divorce wasn’t legal yet so there had to be a whole court case to prove *if* there was abuse, infidelity, or fraud
Marriage was always meant to protect women in the sense that it gave them a home and a name when they weren’t allowed to acquire their own, and it benefitted men in the sense that they frequently acquired assets from their wife’s family. Socially it implied that men weren’t allowed to abandon these women except in extreme circumstances, if they wanted to remain in their social circles good graces. Now women can do their own thing, so marriage is primarily for financial & parenting benefit.