Why do people get married when there are so many contentious divorces?

So many times I hear people state "I never thought they would do this to me" or "I thought I knew him/her so well, but obviously I didn't" when they are in the process of breaking up or getting a divorce after years (and even decades) of being together...I've heard so many horror stories of bitter divorces where the sole purpose of one of the parties is to seek "revenge" and they do not care the extent of hurt they cause their ex or even their kids! How does that happen? How do people who are "so much in love" for so many years, eventually wind up being HORRIBLE to the person they once loved so deeply? What's the point of staying connected to someone in such a bitter way, rather then move on amicably or find peace, particularly when there are children involved? Why does this happen so often? And how do people then trust anyone enough to get married after hearing so many times the "I thought I knew him/her...?" When the divorce rates are over 50% and when most divorces are battles that can go on for a "lifetime" why get married? Why take the risk?