This seems like an invented absurdity, but consider that people in the past few years tried a wide variety of “snake oil” prophylactics/treatments for Covid, including drinking bleach, snorting cocaine, and taking oleandrin (a poison with no known health benefits promoted by officials in the Trump administration), chloroquine/hydroxychloroquine (a malaria treatment), ivermectin (an antiparasitic), etc., even when these had been shown ineffective and were being actively discouraged by health officials. Of even broader impact (hundreds of thousands of extra lives lost in the USA, possibly millions worldwide), many people refused Covid vaccines under a variety of crank theories, driven by a large and organized disinformation campaign. Studying what happened and figuring out how to do better next time should be a significant priority for health research.
We've had enough (and too much) food for maybe a 100 years, even less in some places, and evolution cannot adapt so fast, especially since most fat people die old enough to create new babies before that.
Food was abundant in the US for most people long before that.
It's multiple factors, but they're all mostly easy to observe, measure and identify.
High-sugar, high-fat, packaged foods with preservatives, emulsifiers and artificial colors became cheap, abundant and normalized. These are essentially the perfect cocktail for obesity (by perfectly mixing calories, insulin response, gut microbiome changes, satiety, dopaminergic effects, etc).
Simultaneously fewer Americans are working physical jobs, the jobs that are physical have reduced physicality and society in general has shifted to more car-centric and sedentary leisure (watching TV instead of going fishing on Saturday).
Finally social changes. Women's liberation movement means more dual income households (and single adult households) which means less cooking from scratch and more fast food, packages food, etc. Also strong anti-bullying, body-positivity and self-esteem cultural norms prevent people from getting any peer feedback about their health/weight until it's a doctor telling them that they have diabetes and high blood pressure.
"due to a shorter lifespan of 8.6 years, smokers’ mean total healthcare costs during the entire study period were actually €4700 lower than for non-smokers. For the same reason, each smoker missed 7.3 years (€126 850) of pension. Overall, smokers’ average net contribution to the public finance balance was €133 800 greater per individual compared with non-smokers."