I know this question is somewhat speculative because we know so little about the Fall and our parents - as testified by the many theories about what the images of Genesis correspond to, what the sin was, how much is literal, etc.
For this question, I would like us all to assume that physical evils including animal pain, death, storms, diseases, and the like have been present since the very beginning of the Universe. The Church only has us believe that human death and suffering resulted from the Fall. And this is good, for science shows us that our universe and our Earth go through natural finite processes that allow for decay, death, and the arrival of new organisms.
The question is: Why would God give our first parents the preternatural gift of immortality in the context of a world seemingly unfitting for that gift? The surrounding world consisted of things, such as microbes, storms, predator animals, etc – all of which could otherwise kill our first parents, had it not been for gifts of impassibility (no suffering) and immortality. We know that in Heaven, the world will be such that our immortality in the body will correspond quite nicely to the glorious environment around it.
But why this gift of immortality in such an otherwise contrary environment? Do we have any ideas to why this was the case?
Again, I will not appreciate answers that do not assume what the science tells us. I do not at all accept the view that the Fall of man introduced physical evils such as suffering, deadly storms, diseases among plants and animals, and so on.