Big brands of cars do advertise, their advert is not seen by many because it is done towards a target audience. Big brands of cars do not do mainstream advertisements because their product is targeted towards a particular set of people and not everyone.
They advertise in financial journals, luxury magazines, at car shows and in places where high net worth people are.
Some of their cars do not need to be advertised because they are known to many, we see them in movies and many celebrities use them.