The food changed substantially. Lot's of other stuff changed too, but we're talking about the transition from malnutrition to obesity over a few generations and the make up of what we ingest strongly impacts that.

Mass production and consumerism of food created perverse incentives that resulted in drastic changes to diets - different macronutrient profiles, new ingredients (emulsifiers, artificial sweeteners), and new manufacturing byproducts (residual solvents, pfas, plastic). You can still opt-out of mass produced food and move to the bush to live a subsistence life, and you'll still be ingesting plastic and pfas.

Part of the reason this is so hard to grapple with is that (in the USA) almost no one alive was an adult before the food industry industrialized. I'm 40 and my parents and grandparents rode the initial waves of processed garbage.