What Are You Doing To/With Your Car Today?

Does it not have to hit metal to ground it and turn it off?

Nope
It is a "normally closed" push button type

With the door open, the circuit is closed and whatever is on the circuit will have power to it (in this case, the dome light)
When something presses on it (your finger, or, under normal circumstances, the door) it presses the button and breaks the circuit, interrupting the path and shutting of the dome light