Why can't our immortal souls make our body also naturally immortal?

Question

I have been reading what St. Thomas has to say on resurrection and how the human body will be changed afterwards. St. Thomas talks about how all people, the damned and the saints alike, will become immortals. He says the soul will communicate its natural immortality to the body.

Summa Contra Gentiles, CHAPTER LXXXIX--Of the quality of Risen Bodies in the Lost:  

Now the human body, after the resurrection, will not be transmutable from form to form, either in the good or in the wicked; because in both it will be entirely perfected by the soul in respect of its natural being.

Summa Contra Gentiles, CHAPTER LXXXVI--Of the Qualities of Glorified Bodies:  

The bodies of all men alike will be organised as befits the soul, so that the soul shall be an imperishable form giving imperishable being to the body, because to this effect the power of God will entirely subject the matter of the human body to the human soul.

  Summa Theologiae, (This is from my notes and I forgot where exactly in the Summa this is):  

But in the final state, after the resurrection, the soul will, to a certain extent, communicate to the body what properly belongs to itself as a spirit; immortality to everyone; impassibility, glory, and power to the good, whose bodies will be called "spiritual."

If this is the case, then why can't the soul keep the body from corruption now so that humans become naturally immortal without the need of any preternatural gift to keep us immortals?
I wish to get an answer from a metaphysical and Catholic perspective.

Answer 1

The short answer is that the wage of sin is death (Rom 6:23).

Expounding on this, remember that death entered the world through Adam's sin. Before the Fall of Man, though they were not perfect, Adam and Eve could not die. They had to sin in order to let death into the world. Otherwise, if they had not sinned, their stainless souls could have sustained their bodies indefinitely. Indeed, immortality is one of the preternatural gifts (See the last paragraph on this entry), so we know that Adam and Eve would not have died if they had not sinned.

Because we continue to inherit original sin even now, we all are without the preternatural gifts. This is why we fall into temptation, why we fail to fully grasp things with our intellects, and why we die. In Catholic theology, sins come with both a temporal and an eternal consequence. Although Christ working through the Sacraments restores our souls to life and removes the eternal consequence, we still have to work out the temporal consequence. If we stole something, we have to give it back, for instance, as well as pay the legal penalty for theft. The death of the body is the temporal consequence of original sin, and the "death" of the soul (really eternal separation from God of the soul) is the eternal consequence. The Sacraments remove the eternal consequence, the death of the soul, while the temporal consequence - bodily death - remains.

Answer 2

Why can't our immortal souls make our body also naturally immortal?

The short answer is that God created our human nature (body and soul) and rules the world by divine reason.

Besides that God knew man would use his earthly body as an accomplice to the soul in sin.

Natural and Human Law

Thomas Aquinas, much like Aristotle, wrote that nature is organized for good purposes. Unlike Aristotle, however, Aquinas went on to say that God created nature and rules the world by "divine reason."

Aquinas described four kinds of law. Eternal law was God’s perfect plan, not fully knowable to humans. It determined the way things such as animals and planets behaved and how people should behave. Divine law, primarily from the Bible, guided individuals beyond the world to "eternal happiness" in what St. Augustine had called the "City of God."

Aquinas wrote most extensively about natural law. He stated, "the light of reason is placed by nature [and thus by God] in every man to guide him in his acts." Therefore, human beings, alone among God’s creatures, use reason to lead their lives. This is natural law.

The master principle of natural law, wrote Aquinas, was that "good is to be done and pursued and evil avoided." Aquinas stated that reason reveals particular natural laws that are good for humans such as self-preservation, marriage and family, and the desire to know God. Reason, he taught, also enables humans to understand things that are evil such as adultery, suicide, and lying.

While natural law applied to all humans and was unchanging, human law could vary with time, place, and circumstance. Aquinas defined this last type of law as "an ordinance of reason for the common good" made and enforced by a ruler or government. He warned, however, that people were not bound to obey laws made by humans that conflicted with natural law.