There's a lot of theories about seed oils, PUFAs, microplastics, corn syrup - what if it's just that food costs have gone down as a percentage of income over the last century and it took a few decades for the culture of overconsumption to kick in?

100 years ago people would have spent 25%+ of their income on food, nowadays it's more like <10%. You don't see this in countries where the income is much lower (although they're not eating a western processed diet, so it's hard to decide what's the cause). Medieval nobility were the only ones who could afford to get fat (e.g. Henry VIII) - maybe if the peasants could afford to eat as much as him they'd all be as fat as a modern person.