Wouldn't that reward raising them in a way that increases the likelihood of them supporting you? And/or raising more of them so that the odds are at least 1 supports you?

The problem societies have is reconciling both individual vs societal interest and short term benefits vs long term benefits. I don't see that being solved with any kind of legislation, especially not by a legislature that has to depend on votes today.

As a side note, some places do try to legislate it with filial responsibility laws:

https://en.wikipedia.org/wiki/Filial_responsibility_laws