I don't have a solid or fully-articulated understanding of this; but it seems that incarnation makes choice easier and more possible, compared with living as a disembodied spirit.
To put the same thing differently - the major benefit of incarnation is that agency (i.e. free will) is enhanced - which probably has something to do with the boundaries of 'he self' being more defined.
I don't think there is anything that prevents a spirit from exercising agency; indeed I regard all living beings as having some degree of agency, and I regard divine creation as consisting of living beings (even the 'mineral' world).
But agency is somehow much weaker and slower in a spirit than with an incarnated being - perhaps because incarnation is the physical expression of spiritual separation.
A spirit - who is not spiritually separated from other beings - probably finds agency much weaker and slower to operate. Or perhaps agency in a spirit is more easily confused by 'interference' from the cognitions of other beings?
In other words; a major benefit of incarnation is the enhancement of agency. And this is - presumably - why Jesus was resurrected as he became eternally-divine; and why men are offered resurrection as the path to eternal divinity as sons and daughters of God.
Pre-mortal spirits live as not-fully-separate beings, immersed-in an environment that is permeated with God's goodness. Pre-mortal spirits therefore find it difficult Not be be aligned with God.
The existence of demons (pre-mortal spirits that have chosen to leave Heaven and reject mortal incarnation) seems to shows that strong hatred of God can still be formed and expressed as a spirit. But my assumption is that only the most evil (the most innately hostile to God and creation) of pre-mortal spirits are able and willing to make the choice permanently to reject both God's creative will and the chance of mortal incarnation.
In other pre-mortal spirits - hatred and rejection of God and creation only becomes clear after incarnation - as we observe with the most innately and deeply evil of Men on earth.
After death of the body, those who then reject resurrection and remain as spirits - but self-excluded from Heaven; presumably return to this feeble and slow form of spirit agency, exacerbated by the maiming effect of having lost their bodies.
But are post-mortal spirits necessarily maimed? This is suggested by the accounts of the inhabitants of Hades and Sheol as shades, who do not know their own identity - 'demented ghosts'.
Pre-mortal spirits (before mortal incarnation) are complete beings. Incarnated mortals are also complete being - albeit temporary.
But a spirit that remains after mortal death of the body is probably not complete. After all, the body and the soul are integrated and not separable during mortal life, therefore biological death of the body probably has a maiming effect on the spirit that remains.
Such maimed post-mortal spirits then presumably find themselves in some kind of Hell: a God-hostile environment, permeated with many evil cognitions from other beings.
I therefore think it may be that repentance after death is very difficult and rare for at least three reasons:
1. Being a spirit - hence intrinsically less agent.
2. A maimed spirit - hence incomplete.
3. Dwelling in an evil (God-hostile) environment - living among other beings that have also rejected resurrection into Heaven.
In sum; the spirits 'in Hell' could in principle repent their sin and choose resurrection into Heaven (certainly, God would be delighted if they did so). But in practice, and for practical reasons; repentance is much slower and more difficult, thus less likely, than in mortal incarnate life on earth.
Edited from a comment at William Wildblood's blog