You described the panacea: Eat less. If you eat less than you burn, you will lose weight. This effect is so predictable and reliable that taken to its logical conclusion it results in death. We call this starvation.
And everyone really isn’t different. The only thing different for most people is what sort of diet they find more tolerable. The fundamentals don’t change though.
It's like saying "people are in debt because they borrowed money" - true, but banal and trivial, and pretending it's the whole story and saying nothing of medical debt, student loans or living from paycheck to paycheck and payday loans dayloans is only good for feeling better than someone, most likely while pretending that all people in debt are stupid because they buy too many cars and TVs or whatever.
The whole area (both nutrition and debt/poverty) is very complex, especially when looking at whole society. Thinking in simple caricatures, and dismissing people as stupid is helping nobody.
If you’re an individual who wants to lose weight, focus on calories in and calories out because those are the biggest factors by far and they are also largely within your control. If you want to talk about social problems then ignore individuals and discuss the broader problems that are leading to such a large and consistent calories in discrepancy. In either case gut flora is probably not the major factor and probably not where the solution will be found.
I also didn’t dismiss anyone as stupid and your accusation is out of place.
A lot of people tried that, and it didn't work, but we never got to "why doesn't it work for so many people" because people like you keep parroting "CICO! You're just weak! CICO! CICO!!" even though it helps nobody.
Of course, calories in and calories out are mostly out of your control, but we're past that, since this has now become about moral judgment of obese people.
More to the point, you haven’t said anything meaningful about society’s obesity problem except to assert that everyone with a weight problem is a victim completely lacking in agency.
If a person doesn't eat enough food, the carbon comes from stored fat cells.
There is no panacea for weight loss - everyone is different.
A book is not a source. Providing a book and then cavalierly saying people should read it because it's backed by science is pretty rude.