Quality of food is certainly declining, but we are also seeing weight increases in lab rats with extremely controlled diets, so food probably isn't the entire story. There's no accepted explanation for this, but good candidates that might apply to humans are: subtle toxicity by microplastics, and changes in gut microbiomes.
I don’t think anyone claims that obesity is only achievable with modern processed food. At the end of the day it’s calories in, calories out.
That said, there’s more to the story. For instance, type 2 diabetes in children did NOT EXIST before ~1980, even though there was plenty of abundance and environmental contaminants. This suggest that there is a major issue with the food itself. If you look at where there is the most obesity, within the US or in the world, you also find a strong correlation with processed food. For instance, the Pacific Islanders who suffered obesity epidemics which coincided with processed food replacing their traditional diets.
Quality of food is certainly declining, but we are also seeing weight increases in lab rats with extremely controlled diets, so food probably isn't the entire story. There's no accepted explanation for this, but good candidates that might apply to humans are: subtle toxicity by microplastics, and changes in gut microbiomes.