Why? So people can make each others life more miserable
1) because most people are going to think twice about getting married if they know that getting unmarried will be a huge pain in the ass, and that it might even be a fight to get away.
2) because delay and obstacles to divorce gives the agents of the collective who support marriage and the family time to help the partners overcome the obstacles in marriage which have convinced them to get a divorce.
this may have changed, but in Munich 1990-92 I worked for a guy who was getting a divorce. He was living with his girlfriend, but the divorce itself was expected to take 5 years, because he had kids and because the wife did not want a divorce....that was a good way to handle the situation.