I have seen grotesquely-obese people in their 30s and 40s navigating their way on carts in Wal*Mart. Such people are effectively disabled. I can't imagine anyone willingly eating himself into such a condition.
Death wish?
How about an effective system put into place by the ultra wealthy elites that encourages people to eat low price unhealthy foods that have addictive qualities to them? And yes I'm including those elites that pay lip service to "supporting the poor and middle class" while also supporting policies that have destroyed them while also promoting policies that support the ultra-conglomerates.
If I wanted to believe the worst I would think of pathological obesity as one way to dispose of people that our economic elites consider 'no longer desirable in the workplace'. So what do you do if you no longer need someone to work in your factory but who has no use elsewhere?
Food -- especially high-fat, high-sugar food -- is cheap in America. It's cheaper than a pension or disability payments. If someone with a walrus physique dies of a heart attack at age 48 -- well, that's someone who will never collect Social Security, Medicare, or a pension. The deceased will even supply copious fuel for a cremation and make that cheap.
Of course I hope that such is a wild fantasy. Maybe that is good for a sci-fi story.