God’s simplicity is a matter of logical necessity.
That is, God is simple necessarily. If you deny God’s simplicity then you must explain what joins the separate essential properties of God together into God. And you can’t begin to provide such an explanation without presupposing that God’s parts exist as separate eternal forms or abstractions, i.e., objects that exist independently, distinct from God’s existence, from all eternity.
In other words, you must assume that there are features of ultimate, transcendent reality that exist apart from God’s existence. There must exist independent properties (actual, really existing goods) that come together to make up the essence of the composite God.
If that’s the case then God cannot be the great I AM. We have to reject divine aseity. We have no answer to the Euthyphro dilemma. And a host of other serious problems emerge.
In other words, the reason we must consider God to be simple is that rejecting divine simplicity leads to absurdity.