Because your happiness correlates more with the quality of the relationships you have with your family, marriage reduces your chance of depression and mental health problems and doubles your chance of happiness.
Married men make 10% - 40% more money than single men. Not only do they make more money but they also spend less since items and their expenses are shared with another person. They also spend less money on the more frivolous things because they have someone else to support.
Marriage means more sex which is good for you. Single people go out and look for someone to have sex with but married people don't have to, its more accessible. It keeps you happier and healthier, and boosts your self esteem.
With marriage you always have a friend. You have someone to talk to when you're lonely or upset. You have someone to support you emotionally whether it's sorrow or joy. Your spouse is never far away.
Marriage actually makes your life longer. Marriage gives you an extra seven years of life. Because you don't feel alone and isolated, you are more likely to live longer.