Similarly, people choose to have kids for lots of reasons (family pressure, want to pass on genes, etc.) but society's continued existence depends on people having kids, so it's on all of us to support the people who do have kids whether or not any given individual's motivation is noble.
How might we use the trillions of dollars spent on the military for education, healthcare, retirement and other betterment initiatives for U.S. citizens? If hypothetically all nations agreed on world peace, then having a military becomes superfluous but the would-be soldiers can find other meaningful professions in society rather than being soldiers.
Again I'm not against supporting parents. Just when there's immarginal benefits being conferred, that are not available to non-parents.
In a perfect world, that would work. We don't live in a perfect world, which is why virtually no society has ever disbanded their armed forces and lived to tell about it.
Similarly, in a perfect world, we could treat every individual exactly fairly and never given someone slightly more than they should have. But in this real fallen world we live in, sometimes the best we've got is simplistic decision rules like "support categories of people who do work that is essential for society to survive".
Let's stick to the simpler rationale that some people have kids because they love kids and/or want to pass down their genes.