Ancient people often fell into idolatry. Idolatry is not a mental exercise. There is a man-made idol, usually a statue, and people believe the artifact is a god and they worship the artifact. This is hard to understand for most contemporary people in our times. However, we can view idols as symbols of uncontrollable fortunes. Idols can represent fertility, abundance, victory, etc. Ancient people believed these gods could dispense fortunes at their pleasure. So people offered all kinds of things, animals, and even human beings to idols to have favours from these gods. Therefore, we can also consider idols as the projections of human desires. Each idol can be seen as a representation of a desire or desires. For instance, one famous idol in the Bible is Baal. Often this god is associated with fertility. Then, Baal seems a projection of human desires for prosperity and sensuality. Thus, we can say idol worship or idolatry is, in fact, worship of human desires.
Then, what is wrong with worshipping human desires? As we know by experiences, human desires are in conflict with one another, and human beings are in conflict with one another to satisfy desires. In other words, when I pursue my desires, I fall in confusion within myself with conflicting desires, and I face infinite competitions against others to fill my desires. Interiorly or externally, we will not be fulfilled if we worship our desires.