So the government should redistribute wealth via taxes to incentivize having kids. Why would an individual employer be expected to take on that burden? Not to mention the arbitrage opportunities it opens up.
Because you want society to exist, and keep existing. More concretely: you want to have people around you until you die, and it'd be kind of immoral to demand they kill themselves when you kick the bucket. So they will want people around until they die. And so on and so forth.
TLDR: you want children to exist now.
And that's ignoring how our society works, because the way pensions work you want about 4 times as many children as old people. That's a lot of kids we haven't had, but the alternative to that is way more taxes. Clearly we don't want way more taxes, so 4 times as many kids it is. Except we're not doing that either, and ... uh ... I guess we're hoping God will save us?
TLDR: you want children to exist now. And, actually, you should want the number of kids to roughly double from what we have now (in the US, in Europe, a minimum of 4x is needed)
I mean the saying goes they are the future, and it a very literal sense, they are.