Why does light switch not use ground wires inside of box?
In 1988 switches still didn't require ground screws. Safety protocols have ratcheted up since, and switches installed today do. Someone may have replaced that switch and not bothered to connect it, or it just wasn't used initially to save time. 

If you like, add a pigtail. As Jim Stewart says, "Each switch must be grounded. This could be done with separate pigtails from the ground bundle, but if that is inconvenient then you could use a single pigtail which has a "U" loop around the ground screw of one switch and goes on to the second switch."

The risk is minimal without grounding, though, if the rest of the wiring was done to high quality.