Society needs children to survive. Even if you don't care about the propagation of the species, you will need younger people to run things when you are in your old age (if you want to be comfortable). So in that way we all have an incentive for children to exist (even if you don't want or have any yourself).

As a more concrete example, in the US we need a working population to support the Social Security of the retired population.

When these rules were being created, people assumed that children are better off in home with two married parents and that people are more likely to have children if they are married. So it made sense for the government to encourage marriage with the incentives you mentioned.

Of course, you may want to argue those assumptions now. But that is beyond the scope of my comment.