Blake's myth posits our pre-existence, like Thel in the pastoral Vale of Har; we all choose material, temporal life. That's why we're here-- for a time! Eventually we will return--whether we will or not.
But like Thel the choice was ours; we chose life; she declined.
Why do those in the 'above' choose mortal life? Who can say? Some do; some don't.
Thel explored the option. She found the end of mortal life fearsome. With a screech she forsook the world and presumably returned to Har.
For Blake everything is a man: rocks, clouds, all creatures, the whole Creation"
"Cities are Men....and Rivers & Mountains are also Men; everything is Human, mighty! sublime!" (Jerusalem, Plate 34 ; line 46ff; Erdman 180) Also lilies, clouds, worms, a Clod of Clay.
In Thel we meet the Lilly, the Cloud, the Worm, the Clod of Clay. The last one had this to say:
"...on Thel she fix'd her humble eyes;
'O beauty of the vales of Har, we live not for ourselves.
Thou seest me the meanest thing, and so I am indeed:
My bosom of itself is cold, and of itself is dark,
But he that loves the lowly pours his oil upon my head,
And kisses me, and binds his nuptial bands around my breast,
And says: "Thou mother of my children, I have loved thee,
And I have given thee a crown that none can take away."
But how this is, sweet maid, I know not, and I cannot know;
I ponder, and I cannot ponder, yet I live and love.' (Erdman 6)
Such a beautiful passage! the 'Clod of Clay' is the mother of God's children, 'he that loves the lowly'. God promises to redeem the entire Creation. ("the whole Creation groans in travail ......waiting for the Redemption" (Romans 22).
"Cities are Men, fathers of multitudes, and Rivers & Mount[a]ins Are also Men; every thing is Human, mighty! sublime! (J34.47f; Erdman 180)
And from Milton, Plate 22, 24 line 17ff; (Erdman 117):
"Six Thousand Years Are finishd. I return! both Time & Space obey my will. I in Six Thousand Years walk up and down: for not one Moment Of Time is lost, nor one Event of Space unpermanent. But all remain: every fabric of Six Thousand Years Remains permanent: tho' on the Earth where Satan Fell, and was cut off all things vanish & are seen no more They vanish not from me & mine, we guard them first & last. The generations of men run on in the tide of Time But leave their destind lineaments permanent for ever & ever."
You could construct an elaborate and beautiful cosmology out of that idea:
Jerusalem Plate 99.1; (Erdman 258):
"All Human Forms identified even Tree Metal Earth & Stone. all
Human Forms identified, living going forth & returning wearied
Into the Planetary lives of Years Months Days & Hours reposing And then Awaking into his Bosom in the Life of Immortality."
When we've completely annihilated our Selfhood, our journey is complete:
"When once I did descry the immortal man who cannot die Through evening shades I haste away to close the labors of my day." Gates of Paradise, (E 269)