We know that fast food is bad for us. But fast food companies keep putting the things that we like into it. So a lot of people, when tasting actual, real, good-for-you, food decide that they prefer fast food. Other people are aware that fast food is bad for them and prefer real food. It's a choice that we leave up to the individual. Unfortunately we then allow the fast food companies to advertise so they can affect the choice.
We don't really have an answer for this as a culture. We should make the fast food companies responsible for the harms they're causing, but we don't have a mechanism for that. We could stop them advertising, as some countries have done, but that starts a whole process of questions about what the government can and can't do that ends up in bad places.