Probably because people aren't acting on it, or sticking with it long enough to actually see results.
Especially in today's world when they can just take a pill and loose 10-15 pounds, nobody wants to do something they probably don't like doing (consistent exercise or healthy eating) as part of a short-term or long-term process.
A lot of people, especially those in food deserts or without access to adequate nutrition on a consistent basis (or those who cannot afford quality food, or those unable to prepare it properly) are not going to undergo something where change is gradual versus taking a magic pill, side effects be damned. Sad, but that's the world we live in.
Weight has been increasing for nearly 40 years now in the US among all groups. Trying to point this out as any kind of individual failure just has not, and is not going to work. It's tantamount to telling someone to stop smoking in 1950. Yes, some people did manage to, but the vast majority stopped smoking when their lungs no longer supported them living.
When pretty much everywhere you go is plastered in signs saying "EAT THIS YUMMY UNHEALTHY BULLSHIT" and said bullshit is cheap and highly available don't be surprised when we have society wide problems. Add to that our work practices commonly have us hold down a computer for 8 hours a day, it should be no surprise we are where we are.
A small group that's used a different selection mechanism than the primary population is really a poor metric to judge the majority. It seems likely that we'd find that migrant populations that have to a lot of work to get where they are have a higher motivation level than the general population.
One should look at Asian obesity level growth, then compare the populations you're talking about in particular
Probably because people aren't acting on it, or sticking with it long enough to actually see results.
Like telling penguins they need to flap harder to fly. The literature on diets on abysmal. People lose some weight and regain it, probably as willpower runs out. Look at all these celebrity weight loss shows over the past 2 decades--almost every contestant regains the weight, and these are people who have a lot of money and access to top chefs, trainers, etc.
Especially in today's world when they can just take a pill and loose 10-15 pounds, nobody wants to do something they probably don't like doing (consistent exercise or healthy eating) as part of a short-term or long-term process.
A lot of people, especially those in food deserts or without access to adequate nutrition on a consistent basis (or those who cannot afford quality food, or those unable to prepare it properly) are not going to undergo something where change is gradual versus taking a magic pill, side effects be damned. Sad, but that's the world we live in.