Parents cannot stay with their children throughout their life, because everybody has to go one day. And, without considering some sudden and accidental changes in anyone's life, children would be alive for more than 20-30 years at least compared to their parents.
Considering life as a long journey, and a person cannot go through it alone, everybody needs their life partner, who accompany them in good and bad times; to share the feelings; to love each other; to care each other, and other than parents, only a spouse can fulfil all these requirements.
Marriage is a lifetime bond and needs love, respect and trust to make it a successful one. Considering marriage as a positive outcome, the reasons to get married are:
You can't get old sitting alone in a chair. You need someone to be there for you. There will be someone to take care of you in your olden days. With marriage, you get a companion for a lifetime.
There will be someone, who will stand by you in your good and bad times, who will be a true friend for the rest of your life. A person, with whom you can share everything.
With this kind of thinking Indian parents want their children to be married and lead a complete family life with spouse and kids. Our culture does not call a single person as a family.