For the same reason your taxes pay for schools even when you don't have kids.
Because we live in a society.
If you struggle to understand why that matters without reference to more direct personal stakes for yourself, just know that without a society, the children will grow up to rape you, kill you, and possibly consume you for your protein content.
But I have benefited from public education. Me paying for someone else's education in return is fair.
>Because we live in a society.
"Because we live in a society" is a phrase flexible enough that it can be used to justify anything, including outright theft: "we live in a society; us taking your stuff is the price you have been chosen to pay for your continued belonging". Want to try a little harder?
I stand humbly corrected.