Because you like kids, you want to raise kids, and you want to give over your life for them.
Having them to take care of you is selfish, shallow and frankly vile.
Having kids so you’ll be taken care of in your dotage is NOT the model God created, that’s a man made model that’s wrong in every possible way on every possible level. You should be having kids for THEIR sake not YOURS. Having kids is the ultimate commitment, if you do a good job they will take care of you, but if that’s the only reason you’re having them you WON’T do a good job. That’s kids as props, way outside of God’s design.
And actually what you outlined is commie talk, raising one generation with the goal of them taking care of the previous, that’s the ultimate collectivism. And definitely no way to live.
“Having kids so you’ll be taken care of in your dotage is NOT the model God created,”
I didn’t say that’s the ONE reason. The older take care of the younger, the productive take care of the older/feeble, etc., we all take care of one another.
And God did indeed institute the family, starting with Adam and Eve, Cain and Abel, and it was the only institution for a very long time. Many of His laws concern protecting it (Honor Your Father and Your Mother, Thou Shalt Not Commit Adulter) and there is a whole structure of headship and submission, rights and responsibilities laid out both in the New Testament and the Old.
The family is THE vehicle for social care and continuance.
I respect the right of anyone not to procreate. But even if they don’t, they are part of a family, too! Except in rare instances of being total orphans.