I think it's as simple as the availability and quantity of food in front of us at all times.
We're eating just as often as in the past, but portions (of everything) are so much larger and our bodies will eat more if it's available.
Satiety is always a lagging indicator to stop eating, so by default we'll over eat if we can. Combine this with food always being available and it doesn't take a genius to figure out where the trend is going to go.
This feels even more accurate to me as I've started doing alternative day fasting (because I'm about 50 lbs overweight), but I've been tracking my food intake on days I eat and I'm almost always eating 2500-3500 calories.
Stuff adds up quickly.
* 500 calories for a bowl of cereal with milk
* 1,000 calories for a sandwich from a sandwich shop
* 1,000 calories for dinner
* 500+ calories from drinks
And eating all of that doesn't make me feel stuffed. It's what makes me feel full. Which I know is a good bit more than "just enough" which I've done when I was working out a lot and watching every calorie I ate.