Why is conduit for 120V building power metallic?
The conduit will be connected to earth.  If the insulation breaks down, the live cable will short to earth, drawing a large current that will trip a breaker.

If the conduit was an insulator the fault would go unnoticed.

The cable in the conduit may well be a steel reinforced cable that will have the conductors surrounded earthed steel.  If the live insulation did break down it would would then short to the steel and again trip a breaker, so in this case the insulated conduit would be unnecessary too.