Let's start with your asserted fact and see how valid that is:
(All of our spiritual books claim that, God is NOT Physical but Spiritual(lacking material body or form or substance!)
But unless you toss out the New Testament and the bible from the category of "all our spiritual books", we seem to find this little quibble in the book of John about material body:
John 1:1 - In the beginning was the Word, and the Word was with God, and the Word was God.
John 1:14 - And the Word became flesh and dwelt among us, and we have seen his glory, glory as of the only Son from the Father, full of grace and truth
So, now that we have blown holes in your assertion, what exactly was your question again? Oh, yeah, why do rational people ask for a rational explanation how nothing (no body, form, or substance) can be something (like word becoming flesh and living among us)?
I guess we're just a bunch of nutcases who like to go around making false assertions.