why are tertiary alkyl halides practically inert to substitution by SN2 mechanism?

In SN2 reaction, the nucleophile attached the back side of the carbon reaction site and halide ion acts as a leaving group simultaneously.

If the reactant is a tertiary halide, the carbon reaction site is attached to 3 bulky alkyl group and a halogen atom. The steric effect of the 3 bulky alkyl groups prevent the nucleophile from attacking the carbon reaction site. Therefore, it is hardly for tertiary halides to undergo SN2 reaction.

stearic hindrance - the nucleophile can't get in.