Variety of things, some already covered.
"I have a condition" is a possibility but so vanishingly rare that it gets to be a joke, unless it really is Tourettes of the elbow. It is often also somewhat distinctive.
Some of the cheapest food is so laden with sugar and fat to make up for a lack of taste. Same for drinks which really piles it on if you are not careful, never mind going full American and going with actual fizzy drink as more than a treat. On the flip side if you are working all hours to provide for the kids then preprocessed crap which gets given instead is similarly laden. Fail further and fail to do portion control for said kid and... yeah.
Many of the cuts of meat eaten are also all but devoid of some quite necessary nutrition -- muscle meat is tasty if you can't cook well but you do want some nice organs and connective tissues (never mind the utter insanity that is cutting the fat off) which take a bit more effort to cook properly .
Eat enough of the sugar and fat and your body comes to expect it (whether you want to think you are at the behest of the bacteria in your digestive system or not is up to you, however the evidence is there if you want to go looking at fecal transplant long term results and comparing donors). This makes for a nice vicious circle.
Most people work in offices or boring things today, as opposed to trades and factories, and don't really walk anywhere either (in the US you might even struggle to walk anywhere courtesy of town planning). This goes for kids, which have the added problem of mediocre PE* and "OMG the world is scary for kids so keep them inside**" (this despite it being one of the safest points in history, medicine never better, and kids abducted off the street by strangers is dozens of times less likely than it being a relative to touch them in their no no square).
*when various militaries and medics note that the kids can't run and gain injuries otherwise associated with old people and completely unfit people then yeah.
**would someone let their 10 year old go down the park today? Once standard, now probably not even frowned upon but actually not unheard of for it to have the authorities called on you in some otherwise safe places. What about "come back when it is dinner time/street lights turn on/dark"?
Some have noted the habits of grandparents, parents and beyond, as well as early childhood, also play a role. Studies here on places in deepest, darkest nowhere (Norway being of the more interesting --
https://io9.gizmodo.com/how-an-1836-famine-altered-the-genes-of-children-born-d-1200001177 ) wherein if crops failed one year then you go hungry and the results for children born in famine vs not years being fun. This in addition to those knowing how to cook, or indeed not, being more limited than it might previously have been. What goes for this one I don't know -- surveys are a bit more limited and unreliable.
Start fat and you will probably also stay that way -- between said gut bacteria and millions of years of evolution saying "food is gonna get rare, gobble all you can while the going is good, don't use energy unless you have to" it does take a serious effort and actual physical pain. You also have the added bonus of perhaps never knowing anything else (I had a cold a few years back that knocked the wind out of me so to speak and saw me get out of puff that week running up some stairs and climbing some hills that the weeks immediately before and weeks after were fine... was horrifying) and also it is noted if your peers/family are fat then your chances of it also rise (divorce rates, smoking rates... many things are "socially contagious" for this one, though causation and correlation is a hard one to unpack here even if my gut, pun intended, is not going to go pure correlation for this one -- you eat like shit and deem it acceptable to be fat and it will rub off).
