Why live together before marriage?

Many cultures and religions have arranged marriages where the people really didn't know each other but they have a very high success rate of marriage. The United States seems to be the only culture who has thrown out values and tradition and replaced them an alarming divorce rate. I think it is funny that people use the try before you buy theory yet our grandparents never used that approach and stayed married and committed for multiple years. What gets me is people also throw children from previous marriages into the mix ...lets lug them around from boyfriend to boyfriend or girlfriend to girlfriend until we find the "one" never mind if this hurts the children while this happens...by getting attached. Gee - they lost their parents to divorce and we are going to drag them into relationships with men and women by shacking up