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.
It already is a pain in the ass to get divorced. If you have children, there is a lot more pain than in the ass.
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.
But in your own example, the divorce was already complete except for the legal signature. That wife had to walk around every day chained to a man who was living with his girlfriend. How is that helping her? Does she consider herself free to move on or is she in limbo for five years? What are his requirements for supporting this family? Better for all if she can ditch the bum quickly.