I don’t think people are saying that diet and exercise flat out don’t work, but that they do work, but instead, that the route to weight loss is extraordinarily difficult because the time scales are long (you try sticking to a diet of some arbitrary number of calories for 9 months!) and that your body tends to work against you in many ways- set point mechanisms, etc…
In my experience, the effort is almost an exponential curve; it’s easy enough to buckle down and lose 20 lbs. It’s a lot harder to keep that up for 40 lbs, and it’s a freaking Labor of Hercules to continue it on to 60. It’s possible- I’ve done it in the past, but it’s extremely hard. At some point, you just want to live your life and not be consumed by calorie counting, having to spend a lot of your free time exercising, and not being able to just go have a burger and a couple of beers with your friends, without having to literally starve yourself later, or knowingly deprive yourself of any of that kind of pleasure and camaraderie except for once in a blue moon, because you’re trying to lose weight.
Then once you do lose the weight, you had better hope you had implemented an entire lifestyle change, or else you’ll eat like you did, and gain it back sooner or later. And, if you didn’t like that lifestyle that got you thin, you probably won’t much enjoy the one that will keep you thin.
Basically, I’m saying that it can be done, but it sucks and for a lot of people is unrealistic with all the other demands of modern life. Good luck having small children and having the time and energy to count calories, exercise hours a day, and manage to appropriately care for them.
I think a lot of never-fat people have an unrealistic idea of what “diet and exercise” actually is, and it’s awfully condescending to just go around proclaiming that we’re lazy and if we’d just diet and exercise we’d be thin. It’s just as idiotic and condescending as telling a smoker to just quit, or an alcoholic to just quit drinking. We all know those two don’t work that way, so why does everyone assume that being overweight or obese do?