Why? God is one in substance. Water is all made up of the same molecule - H2O (the same substance). The states describe the relationship between the molecules, not the state of individual molecules. You can not assign a phase to a single molecule of water.
I think there is some an idea that God is a singularity - a unit. I think this is philosophical speculation. God the Father does not have physical substance as far as we know. He does not have a body. But this does not mean God does not have parts an all senses of parts. He has different thoughts at least. We do not have to say God is one in a physical sense or a metaphysical sense. "God is one" is a metaphor, just as "God is love" is a metaphor. I think we are pushing the metaphor beyond what is reasonable or necessary.
I'll let you flesh out what you mean on parts. The Confession is below that says no parts. I'm not questioning your orthodoxy whatsoever but when we start going against the confession, redefining words, or even precising definitions (which is what I think you're doing and is necessary at times), and especially on doctrines such as this we all need to take great care. Not sure I agree with saying that "God is one" is a metaphor...
Chapter II
Of God, and of the Holy Trinity
I. There is but one only,
[1] living, and true God,
[2] who is infinite in being and perfection,
[3] a most pure spirit,
[4] invisible,
[5] without body, parts,[6] or passions;
[7] immutable,
[8] immense,
[9] eternal,
[10] incomprehensible,
[11] almighty,
[12] most wise,
[13] most holy,
[14] most free,
[15] most absolute;
[16] working all things according to the counsel of His own immutable and most righteous will,
[17] for His own glory;
[18] most loving,
[19] gracious, merciful, long-suffering, abundant in goodness and truth, forgiving iniquity, transgression, and sin;
[20] the rewarder of them that diligently seek Him;
[21] and withal, most just, and terrible in His judgments,
[22] hating all sin,
[23] and who will by no means clear the guilty.
[24]
III. In the unity of the Godhead there be three Persons of one substance, power, and eternity: God the Father, God the Son, and God the Holy Ghost.
[38] The Father is of none, neither begotten nor proceeding; the Son is eternally begotten of the Father;
[39] the Holy Ghost eternally proceeding from the Father and the Son.
[40]