You're giving the marketing department way too much credit in my opinion.
Pretty much all of these highly processed foods are highly processed in order to reduce cost in some way (e.g. increasing shelf life, decrease storage requirements, use a substitute ingredient, remove the need for some other product, etc). All the finicky bits of producing food (e.g handling ingredients that are much less shelf stable than the final product) and abstracts that away to some factory somewhere. All of this seems like you're getting something for nothing if you don't know it's unhealthy.
The technology and processes used to create stuff that is recognizable as "modern industrial food" were mostly developed and matured over the late 1800s and early 1900s. For reasons that should be immediately obvious cheap and shelf stable became highly sought after traits for ingredients in the 1930s and 1940s. Likewise a generation of people grew up seeing their parents shoe-horn products like Crisco into use cases formerly reserved for other more natural ingredients. Considering that they grew up on it it's no surprise they stocked their 1950s and 60s cupboards and pantries with the sorts of products that they were familiar with from their youth.
Of course marketing is icing on the cake but things, like cooking habits, that you generally learn from your parents are generally resistant to fast change without some sort of strong outside motivation.