Because "marriage" or "relationships" in general are all based on human emotions which changes easily. 

Unlike relationships, marriage is legally binding and holds value in court. You can not just simply walk out of a marriage without having some consequences or troubles in return. That is why, people are more picky and testy on their partners. I personally don't think that they are not scared, more like they are trying to find a person that will be worth it to suffer the pain.