I would guess because the spirit is the epitome of the person. Our spirits are dead before coming to faith though. So where is the spirit of a man before coming to saving faith?
Lets look at "dead" spirit.
That is taken from Genesis, 1, ...
"and you shall surely die"...
So, with God, all things are Spiritual and Covenant, and Light, and Life, and Truth, and Righteousness, and Love, and Holiness..
The Life in Christ is : Eternal Life
Jesus Himself is Eternal live, and when we have Christ in us, we have Eternal Life.
The spirit in a person that is "quickened" is born again.
So, The Bible says..."dead", regarding Adam's, spirit, but it didn't die.. It was separated from LIFE< that is In God.
So, when that separated from God spirit that is in US, is born again, it is restored into spiritual union with God, and He is Life.
Jesus in John 14:6, says..." I am The Way.. The Truth, and THE LIFE"..
Jesus said... I have come to give you LIFE and that more abundantly.
Jesus said......"all that believe in Me, i give unto you ETERNAL LIFE, and you shall never go to Hell (perish)""
John said, "you can know you have eternal life" as that life is Jesus, and being "in Christ". = born again CHRISTian.
See that LIFE?
Adam became spiritually disconnected or separated from the LIFE that is In God and Christ, when he rebelled and ate the Apple.
The bible refers to this as a spiritual "death"
Its not a spiritual death, its a spiritual Separation from God, caused by SIN.
We all have this, same issue, and that is why we go to the Cross, and find forgiveness of all our sin, in The Blood and Death of Jesus, so that God can then give us the new birth, born again.
This is to be spiritually rejoined to God's Spirit, as "GOD...is
A Spirit"
To have eternal life, is to be spiritually joined to Jesus and God the Father who have no END.
"born again....SPIRITUALLY", simply means that we have been returned to the same Spiritual Union with God, that Adam had before He was spiritually separated from God, by his SIN.
See it?