The only problem with the ‘theory’ that all fat people are fat due to the crap they eat and the lack of exercise is that its probably not actually true.
Yes, it is a more complex issue than just bad diet and lack of exercise in some cases. But these two elements are key factors in the majority of cases.
My belief is that it’s the lifestyle changes post WW2 that have generally made everyone less active, by design or by choice, plus the increase in processed and ‘junk’ foods.
As an example, when I was growing up in the late70’s/early 80’s I walked or got the bus everywhere with my mum, there was no dishwasher, no microwave, no laptop, no Wii/Playstation, TV was limited, we played outside at almost every opportunity, all meals were home-made from fresh stuff … overall we were far more active and ate ‘proper’ food … and the key thing: my parents made sure I had a good diet and didn’t just sit around on my ar$e watching TV – they encouraged me to get outside and do stuff, took me cycling, hiking, cricket/football in the park etc etc …