That's because their outer electron levels are full. There is no room to take in extra electrons from other elements and they have no extra electrons to spare. All of the Noble Gases, except Helium, have 8 electrons in their outer electron shell. Helium has two electrons orbiting its nucleus. Metals often release electrons from their outer energy level.
Metals conduct heat and electricity easily.
The only two elements that are liquid at room temperature are bromine and mercury.