Very often actually it's unrelated to eating to much and doing no physical activity (at least wrt. most obese people I ended up knowing, context: Germany/Berlin).
Most obese people I ended up knowing did had a rather controlled diet and did more sport/physical activity then your average person.
So why where they obese? Some combination of:
- They had problems with their metabolism, potential had to take medicament too.
- In their childhood they ended up getting fat due to a varity of aspects and even with strict diet and sport had a hard time of losing their weight (once you where fat enough for long enough time your body gets used to it and as such is much much more prone to add fat to your body when you eat etc.)
- there are other reasons too for other people but not people I did know close enough.
Either-way I don't know about the US but in Germany/Berlin "minimal to no physical activity" is not often the case and neither is missing discipline or laziness.
For many obese people telling them to just do more sport and eat proper is like telling a depressed person to just stand up in the morning and go out or a person with social anxiety to just leave the house. I.e. impolite, discriminatory, mean and far of reality.