Where is Paul getting this from? Covetousness is idolatry sounds nice, just a pithy saying of Paul? A conjecture of biblical logic? It is much more, it is a summation of the Law. Get this:
Consider the foundation of the Law, the Ten Commandments Moses brought down from Sinai. Commandment Number one is:
“You shall have no other God before me.”
The last of the Ten Commandments is:
“You shall not covet.”
God starts off and ends with a main concern of His: don’t want anything more than Me. No matter what it is, if it’s not Him you’ve fallen into sin.