Why is covetousness called idolatry?

As the Apostle teaches: “Put to death what is earthly in you: sexual immorality, impurity, depraved passions, lust and greed (which is idolatry). Because of these things, the wrath of God is coming” (Col 3:5–6; see Q043, Q486).

Covetousness (greed) is a disordered desire for created things, according to the lusts of a darkened heart (Rom 1:21; see Q043, Q492):

Take heed and beware of all covetousness; for a man’s life does not consist in the abundance of his possessions… ‘Fool! This night your soul is required of you; and the things you have prepared, whose will they be?’ So is he who lays up treasure for himself and is not rich toward God. (Luk 12:15–21)

Like all the Commandments, the Second has to do with contentment: God’s creatures are to be content that the invisible God and Father has fully revealed himself in his Only-Begotten Son, whom we have seen. This contentment enjoins restraint and thanksgiving: “how you turned away from idols to God, to serve a living and true God and to wait for his Son from heaven. God has raised him from the dead, Jesus, who delivers us from the wrath to come” (1Th 1:9–10).