100 years ago, there were virtually no obese people. Now, there are. What happened?
One thing that changed is that economic growth made food much cheaper and better-tasting. People also probably get less exercise now. So, the theory goes: some people in this environment overeat, and become obese.
This theory does nothing to answer a crucial question. What is it that causes some people to overat and some people not to?
Many normal-weight people, myself included, aren't trying to be normal weight. We just are. My freezer is stocked with ben-and-jerry's, and I eat pop-tarts for breakfast. I sit at a computer all day when I'm not laying in bed. But after a meal, I get full, and I stop eating. The amount I naturally eat and exercise, while expending literally zero willpower, leaves me at a normal weight.
The typical obese person is not like this. This is not based on any evidence, but in my personal observations, obese people are hungry. I think this obvious fact is ignored because it has negative connotations. It's like accusing them of the biblical sin of gluttony, or making jokes about how fat people just can't get enough cake.
It's an immense privilege I have, to have my zero-willpower state be skinny. If I eat pizza for lunch, I crave salad for dinner. If I deliberately gain weight, for example because it's january so I'm at the gym, I'll lose it as soon as I stop trying. Why did I get blessed with this trait, while so many people suffer the opposite?
I don't think anyone knows, but the good news is that there's an FDA-approved drug called Semaglutide that can induce this in anyone, to some extent. An obese person can expect to lose about 10% of their body weight by taking Semaglutide.
Also, Tirzepatide is now available, and it's even more effective.
Semaglutide is a GLP-1 agonist, and Tirzepatide is a GLP-1 and GIP analogue.GLP-1 is Glucagon-Like Peptide 1 and GIP is Glucose-dependent Insulinotropic Polypeptide. They both cause weight loss via appetite suppression. If I understand correctly, they interact with the process your brain normally uses to tell that you're full.
On the horizon are GLP-1/GIP/Glucagon agonists that might work even better. In addition to suppressing appetite, they seem to incraese the amount of calories burned at rest.

Testimonials

I just searched on google for people talking about Semaglutide on Reddit. Here are the first two posts I found. (You might need to zoom in, sorry.)

Is it an exaggeration to say it's changed the world?

I don't think so. I think obesity is one of the biggest problems in society. And I think one of the reasons it's been hard to solve is because many people blamed it on the moral failings of obese people. Now, you have to wonder if the only difference between skinny and obese people was their sensitivity to GLP-1. (Or maybe not that exactly, but possibly something chemical and easy to fix.)