It is a puzzle why humans need to be born, die and be resurrected before attaining eternal life. And why even Jesus also needed to do this….
Clearly, this fact implies that the actions of our creator are constrained – in time and space – because it is necessary that this process actually happens requiring bodies and duration. (i.e. God cannot do anything instantly – perhaps especially where he is dealing with agents. Some things must be done via stages.)
One way of understanding may be that incarnation is (for Jesus as well as ourselves) into actual, ‘biological’ human families – and Mormons recognise that this is also our eternal destiny.
So; we were premortal spirits who were (in a sense) inserted into biological matter (derived from our biological parents) when we were incarnated. And therefore there is a tension, and an unintegratable quality about our union of spirit and body, during our mortal lives.
This disunity of spirit and body is solved by death – by the death of the biological body, followed by its re-making in a perfected form, from the surviving spirit. My assumption is that the process of resurrection is not possible until the spirit has experienced life ‘in’ a body – and only after this experience that is it possible to make a new, resurrected, perfect and therefore immortal body – using the experienced-spirit as a ‘template’.
In sum – the pre-mortal spirit cannot be a template for resurrection, because it lacks experience of incarnation; and only after incarnation (and this applies no matter how briefly incarnation is experienced – including those who die in the womb, or at birth). But after incarnation is experienced, the spirit is a suitable template for an eternal incarnation.
The temporary incarnation of mortal life makes the template for the permanent incarnation of immortality.