So productive single people are not in the interest of society at large? They don't also procreate? They don't also provide stable homes in which children are raised?Why should we shower riches on people who get married and thus punish people who do not get married? ...
Because it is in the interest of society at large to encourage procreation and stable homes in which to raise children.
I know plenty of married couples that do not have children. I know plenty of marred couples that no longer have children in their home. I know plenty of married couples who do not have stable homes.