Why do giant firms like Coke and McDonalds still advertise?

i believe yes, because they have to be always on top of the sales game, even if they are already very well known brands. they do have competition still and if they show some slack, they would be eaten up by the competition.

As life is like a cycle, everyday could be first time for someone. Maybe the child of strict vegetarian parents who has just grown old enough to make their own decision, may not be aware of such brands, or immigrants or anybody who is just beginning to face the world. These companies are not taking it for granted the "everybody knows this" kind of mentality. The complacency could mean the difference between rebuilding and maintaining brand awareness in the future.