Food got cheap, people got rich, and humans are designed to eat whatever they can see.
200,000 years of living feast to famine tells you "if can get calorically dense food, eat as much as you can, you'll need it later", except your lizard brain never gets the "there's no famine later" message because it's dumb.
Also obesity and wealth are typically inversely correlated within developed countries.
There's more to the story.
And (as sibling comment says) people of more means (say upper-middle class) aren't more obese than poor people. It's more like the opposite.
This is just the myth of the pleasure-button rat extrapolated to humans, which has been debunked in any case.
Americans in the middle income quintile already had easy access to more calories than they should eat in 1960. But boiling another pot of potatoes wasn't as fast or delicious as microwaving another frozen mini-pizza, so there were fewer temptations to overeat.