This mindset drives me nuts. It's like the rhetorical opposite of victim-blaming. It takes something that is within an individual's locus of control and pretends they have zero agency in the matter.
Last week I finished a two month diet where a big chunk of my weekly calories and nutrients came from cheap staples I prepared myself (specifically brown rice, black beans, steelcut oats, spinach, and eggs).
Aside from the food, the cost was 1. watching some ads in the free version of the calorie counter app I used to make sure I was getting the nutrients I needed and 2. ~$30 for a food scale so I could be precise about what I was eating.
Circumstances make it easier to be unhealthy but what I did is attainable by the vast majority of obese people.
It's complicated because it's both, with varying levels of influence.
Obviously, it cannot be 100% on the individual. Because then, how did we get an obesity epidemic? Did people somehow, magically, become more lazy since 1970? That doesn't sound plausible to me.
It's systemic in nature. Consider tobacco use, a problem we've largely solved in the US. We went from something like 50% of people smoking in 1960 to about 10% now. In young people, the results are even more drastic. It's sort of magic - a reverse obesity epidemic.
How did we do this? A combination of things. Of course people worked very hard to quit, but they also got PSAs and their doctor's helped them. And then we made it much, much harder to smoke.
The thing is, people are creatures of influence and habit. Much of what we do is because it's low resistance. We've allowed obesity to become a systemic problem because of our food available, our culture, and our lifestyles.
It's not that some place like, say, Paris is healthy. But it's a lot easier to be accidentally not-obese in Paris, France than in Paris, Texas.
The question of how someone who wants to lose weight should do so doesn't really have much to do with why people in general are more overweight than decades ago. People in the 80s and 90s had a lot of processed foods, didn't generally use food scales, and calorie counter apps hadn't been invented yet.