Resistive material, resistive absorbes energy and turn it into heat, high resistivity limits the current so it can damage anything.
Let's say your part inside the bag is charged with 10kV respect to you, like the plates of a cap, then it conducts slowly dissipating that energy, so for the time you are taking the part out of the bag it's at the same potential you are. If you are grounded to start with as all your equipment and desk there is no more charge to damage anything.
Anti static bags have a inner conductive layer so external spikes can't penetrate and damage parts, if only a high resistivity layer is present like in static dissipative bags external charges can penetrate directly into the part and damage it anyway. Dave has a video demostrating this with a ESD gun, frying components in pink bags while the ones in the grey bags suevive without much trouble.
JS