Demand for food is not infinite (maybe the word you were looking for was inelastic?). And even if you don’t need ads to create demand for “food” that is a broad category with many competing products from many competing providers.
Interestingly the food industry has perverse incentives ... because humans have a maximum limit of food eaten during the day, big companies like Coca Cola can only grow by encouraging people to eat more.
Since the more people eat, the more they spend on food, the more revenue generated.
Therefore food companies are incentivized to produce highly palatable food that trick the brain into overeating. One common strategy is to combine sugar with fat (think donuts), bonus points if it has caffeine too (chocolate with milk). Such combinations are not very natural. You won't find in nature foods that are high in both sugar and fat. And this kind of processing matters. Think of the difference between cocoa leaves and crack cocaine.
And now that we have an obesity problem, the same companies also sell products for diabetics, or diet products with "zero calories" that are highly processed and may contain substances that are damaging to our gut or general health. It's very profitable to create a problem and then to provide the solution too.
And unfortunately you won't see ads for whole foods.
But you will see whole foods vilified periodically (e.g. starchy plants, meat, etc), with ingenuous food companies jumping to the rescue with highly processed stuff. The ongoing race for "fake meat" makes me cringe.
Demand is not infinite. The demand is distributing among a wide range of available choices, advertising aims to change that distribution towards the advertisers products.
