For 3 reasons. First, on the part of the first cause: creation does not spring from any shameful defect in God, such as a potency that would be actualized through the world or dissatisfaction which the world would help alleviate.
Second, on the part of the effect: while God may be a some kind of utilitarian who creates happiness-for-humans, His main task is rather to create humans-who-seek-happiness and do so ex nihilo. Surely, it is good for the universe to be.
Third, on the part of the mode of causation, viz., self-diffusion of divinity. This is a sort of artistic creative self-expression of God that is neither physical nor teleological. God is Light that would burst Him asunder, if He were not infinite. This Light’s impulsion is to create.