How about looking at it like this: Grace, which is imparted at Baptism and eliminates Original Sin, is not natural to humans. By its very definition it is something extra, the Divine Life added on top of our human nature. What is passed on from the parents is human nature, not Divine Nature, and therefore Grace is not imparted (and again, it is Grace that eliminates Original Sin).
Had Adam and Eve not sinned, Grace would be passed along by Divine Providence, not by humans themselves, but as a kind of “conception gift” from God. Since they sinned, however, we are left with only unGraced human nature at conception, and God has instituted a “second birth” of Baptism to aquire the gift that was originally intended to be passed along at conception. It would still have been Grace, a Divine and not human gift, but the timing of its passing on would have been much different.
So, to recap, Adam and Eve (and all their descendents) can only pass on what is natural to them; Grace (Divine Life) is simply not mankind’s to give. So parents who conceive a child give the child what is their own, and that is human nature which happens to be fallen since the sin of Adam and Eve. Even if the parents are given Grace through Baptism, that Grace is not theirs to pass along to their children, but still only God’s since it is His Divine Life and it is above and beyond our human natures. This also allows God to reinforce the notion that Grace is indeed Divine Life, and not merely human life, and that we are saved only by Him and not by ourselves.
To use a worldly example, if I have a house and my friend has loaned me all of the furniture, I can sell the house but the furniture does not go to the purchaser. The house is mine to give, but the furniture remains my friend’s to give or take back as he wishes.
Peace and God bless!