It is not necessary for us to know, but it is a legitimate question to ask why God wanted us to become exalted to divine status, why he wanted to raise us to be Sons of God? And how the conditions on earth fit with the answer.
One answer is that Heavenly Father wanted 'friends' of the same kind as Himself - in other words 'grown-up' friends - as well as child-like children.
He wanted at least some of us to be able to progress to a level where we could engage with Him at a higher conceptual level than was at first possible - God wanted some of his children to mature to spiritual adulthood.
The plan of salvation was therefore designed to enable some Men to be raised to divine status so that Heaven became to some extent an 'adult' society, instead of purely a society of Heavenly Father and his many little children.
(To be blunt - sometimes God yearns for some adult company!)
We can perhaps feel the basic truth in this because we each contain a divine spark from God, and may therefore partially identify with God; and introspectively can perceive this want - this need - in God.
Many Christians will not allow that God has wants or needs, and assert that God is utterly autonomous, impassive and so on. But there is a lot of scriptural evidence that He does have desires, passions and emotions. Indeed a Christian God must have (and be) love - and love is a dynamic emotion, implying change and wants.
Having decided to make a scheme of things in which Man might be educated and grow to divine status, the earth was created and Man was set upon the earth.
The nature of the earth is here assumed to be very different from Heaven - on earth the perception of time is slowed-down, and life proceeds in a manner very unlike Heaven.
In Heaven all is swift, frictionless rapidity and all is bathed in the Glory of God's presence.
In Heaven our will is powerfully shaped by God's will - not irresistibly, but it is hard to resist (and there is not much perceived time to resist) being in total harmony with God - and the relationship tends to be one of childlike loving obedience.
This environment tends to maintain spiritual immaturity - childlikeness. Which is not bad, not at all (children are wonderful!) - but for children to grow toward adult maturity is perhaps better, and what God wants - at least for some of His children.
So, this joyous Heavenly environment is poorly suited to spiritual growth. There is spiritual growth no doubt - possibility of autonomy - but the environment is not conducive to it.
The presence of God is overwhelming, and the frictionless speed leaves little space for consideration.
On earth, in mortal bodies, all is slowed-up.
For example, thought and action are distinct, so there may be first a temptation then space to decide whether or not to yield to that temptation.
There is an inspiration to do Good, create beauty, and then space to decide yea or nay; there is a point when we may choose whether to tell the truth, or not.
And God is not locally present on earth - so we are 'left on our own' with such problems - and can distinguish our will from God's will.
An analogy is that children must spend some time out of the continuous contact with their parents in order to mature - children must themselves learn to overcome temptations even when there are no immediate parental sanctions; they must be able to live well without the immediate incentive of parental approval.
This will inevitably be a matter of trial and error, success and failure; of practice, of habits.
Independent, we may develop strength and good habits or degenerate to weakness and bad habits. But the risk is intrinsic and necessary to the benefit.
That is why God is not present with us on earth.
So earth is well designed for spiritual progression; and is a better place than Heaven for rapid theosis - due to the earthly slowness, friction, distinction of choice from action; and because we feel that we are (by and large) on our own, and must make our own decisions.