Governments regulate a wide variety of private relationships (homosexuality is often treated as a crime, and sexual relationships outside of marriage are discouraged if not actually proscribed), but are unable or reluctant to regulate family size, even indirectly by a system of economic rewards and punishments. This is related to the concept of the sanctity of the family and the awkwardness of governmental interference on such intimate levels of interaction. Leaders in many states (including those with high rates of unemployment and general conditions of poverty) associate population growth with national potency.
Even China, the world's most populous country, was unable to maintain its 'one child' policy, and had to discontinue it after it proved more problematic than advantageous.