Both of those until very recently were extremely expensive to the point of luxury for many, as well as much more difficult to effectively use a lot of cheap and easily.

Effectively zero people are chugging bottles of Olive Oil they buy from the corner store.

There just is no comparison whatsoever to modern processed foods set for immediate consumption and tuned for maximum taste and pleasure response.

Edit: to refute some other points downthread.

> the cost of edible oil has not been a bar to access to calorie-dense foods for at least centuries if not millennia

I believe this to be false. Even my grandparents saw things like olive oil and real butter as luxury items. Crisco started to exist because it was cheap. It took hours of work to turn these items into palatable calorie-dense foods, if that was even the outcome.

Today? The average working class person goes out to fast food multiple times per week. This was a luxury even in my time growing up. Culture has simply changed, and (bad) food has gotten to be incredibly cheap and easy to access.

> it seems to closely follow industrially-mass-produced foods, but the relevant feature of those foods is clearly not that they are more convenient or higher in calorie density than a can of crisco or an amphora of olive oil; they aren't

I'm not sure how you could come to such a conclusion? That is pretty much the sole feature of those foods. Readily available highly-addictive and highly calorie dense foods fit for immediate (or near it) consumption. No magic woo-woo explanations needed. We simply teleported society into a place literally surrounded by cheap easy to access food typically hundreds of feet away at any given time and then wondered what happened to "willpower" as if it ever existed in such a manner.