Can you point to any legislative history that supports your point? When these exemptions were enacted around 1913 they said it was done based on a rough calculation of ability to pay. First there were personal exemptions for spouses and they later added dependents too. It wasn't because they wanted to encourage large families.
How does charity point to ability to pay?
From what I learned in my tax classes a lot of it is designed based on ability to pay and a lot of it is based on things deemed good for society, like charity, home ownership, marriage, and children.
And of course lobbying plays a role too.