It's more expensive to buy and prepare fresh food than shelf stable ultra processed foods and requires more time. Poor people have access to 'poor' calories. I would wager that children would also inherit the eating habits of poor parents.

https://pubmed.ncbi.nlm.nih.gov/20720258/ https://pubmed.ncbi.nlm.nih.gov/14684391/ https://pmc.ncbi.nlm.nih.gov/articles/PMC4254327/