Prior to the 20th century (depending on where you live, but this is for the areas hit with obesity epidemics):
1. You had to walk most places (often miles a day).
2. You had to physically labor. Most people didn't have desk jobs or the period's equivalent.
3. Food was relatively scarce, and harder to transport.
4. Social activities (we are social creatures) required physical effort (walking to, dancing, walking around). We can now be social while being remarkably physically passive.
5. Entertainment options were more scarce, so boredom meant you did something. Now, you can entertain yourself by sitting on a couch or at the desk playing a video game or watching a movie. You could make a case for books being similarly physically passive, but that requires widespread literacy and access to books which wasn't true of the general population.
The expansion of ranching and farming in the great plains of the Americas was sufficiently bountiful to lead to the first 'health food' crazes of the early 20th century--more people had enough to eat, and increasing automation was already reducing labor. So the roots of our current problem were already being planted, as it were. But in the last 30-40 years, there's been a noticeable uptick in average caloric consumption, even more reduced activity, and more of those calories (at least in the US) coming from processed foods with ever-larger quantities of sugar.
One of the likely contributors is the unhealthy long-chain long-shelf-life fats of most prepared foods, which didn't arrive until the 30's and 40's.
Given that most people don't make the time to make fresh meals from scratch, they're getting fairly unhealthy food even if they cook at home. (And many folks short-cut with fast-food options, which are usually worse.)
For having lived in several Asian and European countries, I can claim on the basis of anecdotal life experience that it has a lot to do with the local culture. When I first came to America I was (and still am) shocked by the way people related to food and the ease with which you could fall in a sedentary lifestyle.
1. You had to walk most places (often miles a day).
2. You had to physically labor. Most people didn't have desk jobs or the period's equivalent.
3. Food was relatively scarce, and harder to transport.
4. Social activities (we are social creatures) required physical effort (walking to, dancing, walking around). We can now be social while being remarkably physically passive.
5. Entertainment options were more scarce, so boredom meant you did something. Now, you can entertain yourself by sitting on a couch or at the desk playing a video game or watching a movie. You could make a case for books being similarly physically passive, but that requires widespread literacy and access to books which wasn't true of the general population.