(This is a big question - a natural question; and one for which I haven't been able to find a ready-made answer that addressed and answered it, square-on, to my satisfaction.)
Why mortality? Because it is necessary. An immortal resurrected Man cannot be achieved without a 'transitional stage' of living a mortal life in a mortal body.
This implies that reality is constrained; and represents a major limitation in the power of God the creator.
God cannot create a Man in an everlasting resurrected body without first going-through the phase of mortal life.
Mortality is therefore a consequence of God's creative constraints - in a sense, a measure of God's weakness in comparison with that 'omnipotence' which has traditionally, but wrongly, been imputed to God. Therefore, the many imperfections and incompleteness of mortal life
(disease, ageing, death...) are intrinsic to the fact that mortality is a
consequence of limitation.
If God really had been omnipotent, and assuming for a moment that such an abstract imputation makes sense (which I don't accept) there would have been no need for mortal life (and BTW no need for Jesus). This is a flaw in all theological schemes entailing divine omnipotence - whether monotheistic or polytheistic: mortal life is a superfluity; at best a waste of time and at worst suggestive of a deity that is indifferent to suffering.
This explains why - for so many billions of people in history, so far as we can infer - mortality has been such a brief affair; hardly more than the mere brief establishing of life, of incarnation, of embodiment. Probably most Men have died soon after fertilisation, or as embryo, foetus, newborns, in infancy...
But not all; and most Men nowadays survive for much longer than mere incarnation. Why? Because God has made a virtue of necessity; where that is helpful to us; each as an individual.
God made this life, this world, as a bespoke place of learning for the longaevus such as you and me (meaning the long-lived: that is, those who live beyond mere incarnation, especially after birth); individually-tailored to fit the needs of many millions of individual Men.
So - Mortality could-have-been a very brief phase; and for many it has been: that is, they simply incarnate and die, with the possibility of resurrection. Such is the fate of all Men.
But the extended and complex world, the varied lives of individual Men - all this is for a different purpose than the making-possible of resurrection. It is, indeed, for many individual purposes - reflecting the unique nature of circumstances of each human.
And this second purpose is to learn from our mortal experiences (including the experiences of mortality - disease, ageing, death...); to learn lessons that will be helpful to our experience in the eternity of resurrected life everlasting...
Also, mortal experiences that may help make possible the choice of life everlasting, which is called salvation, among those who otherwise would have rejected it