Tell them you'll start driving a car to work when they pay you enough to afford one.

Tell them you'll start driving a car to work when they provide you with a company car.

Tell them you'll start driving a car to work when they provide you with free parking and gas money.

Tell them to you'll start driving a car to work when everyone else who takes public transportation or walks starts driving a car to work.

None of the above are true, of course, but it will shut them up pretty quickly.