One reason Americans are fatter than ever might seem to be sloth, except for the fact that 60% of the population worked in both 1979 and 2022 but the average work week dropped by just one hour over the period

According to the CDC, obesity is most severe among those of late middle-age, so if you can prevent it before then you are more likely to escape it later:

The prevalence of severe obesity was highest among adults aged 40–59 compared with other age groups.

Peak Baby Boom turned 40 in 1997.

Americans worked 32.61 hours per week on average in 1979 and 31.57 hours in 2022

All countries in 2016 with 30% of population or more obese

60% of the US population was working in both 1979 and 2022