The basic reason is that your body can increase or decreased its energy expenditure drastically in response to increased or decrease consumption of calories.
I'm reading The Obesity Code by Dr. Jason Fung right now and he presents a very strong argument that the cause of obesity is insulin resistance.
In the book he cites both overfeeding studies where subjects were fed very large numbers of calories but did not gain weight, as well underfeeding studies where subjects ate only 500 calories a day but didn't lose weight.
After reading this I'm pretty damn convinced that the diet problems facing us are: highly refined carbohydrates, sugar and sugar substitutes, and alcohol (I haven't come across this in his work, but if you look at the stuff by Dr. Lustig you'll see sugar and alcohol are incredibly similar metabolically)
Think about this: for most of human history we did not count calories and yet we were not obese.
It's the highly processed western diet of breads, cakes, cereals, soda, etc that are the culprit.
I see he's written a book. I don't see any peer reviewed papers or actual scientific studies that have been reproduced by others.
I can't find the exact part of the book where he mentions the guy who ate 500 cal/day and didn't lose weight, but when I find it again I'll note it. This person did have serious hormonal issues, though. Most people of course, 500 cal a day and also exercise weight will come off, how permanently is another question.
I think I saw somewhere on hackernews a calculation that for only maintaining you body temp it takes 800-900 kcal. That is not even taking into account breathing, blood circulation, etc.
Then a few months ago I started following some fitness YouTuber's advice, which mostly boils down to "count calories and consume less calories than your body uses". I started a basic course of counting calories and eating at a deficit, and dropped ~7kg in a few months (and hopefully am still going).
It wasn't even that hard - mostly counting everything and cutting out some of the stuff that is really high in calories. I still ate fairly well, e.g. I'd eat my favorite hamburger - just would skip the fries to be under the calorie count.
Of course I've only done this for a few months and it's hard to say if my body will eventually force me to expend less energy.
I take comfort in the fact that a lot of bodybuilders go to extreme lengths to drop down in body fat, so they've done anecdotal research on this area a lot. And most of them are not following intermittent fasting, or keto, or anything like that - they're just reducing calorie intake when needed, and going up and down in weight fairly consistently.
congratulations on your cut
This is coping, losing weight to a target healthy weight is always good,
If he's jacked, then yes of course he doesn't need to lose weight. That is a tiny proportion of the population though.
Obviously "always" is wrong. Almost nothing is absolute.
Bingo.
I have since gained weight and now have life insurance.
Being underweight is highly correlated with poor health outcomes in seniors. (https://pubmed.ncbi.nlm.nih.gov/16886095/) Obesity is obviously bad there, but being underweight is just as dangerous.
Pretty decent chunk of the world is seniors these days.
Studies on actual cause has shown the opposite (eating to the point of being underweight raising lifespan).
Excluding the fact that average height has increased gradually over time.
It was lowered in July 1998 ("overweight" was reduced to 25 from men: 27 and women: 28)
https://www.nytimes.com/1998/06/04/us/us-to-widen-its-defini...
https://well.blogs.nytimes.com/2014/04/14/a-number-that-may-...
Anorexic is not healthy. Underweight within reason is. In fact the best way we currently know to live more is to eat much less...
https://gizmodo.com/how-living-near-the-starvation-point-can...
* Most layman diet advice is awful. There is scientifically backed advice. I think most people could understand it. It seems to not be common knowledge.
* A lot of people "backseat drive" other people's diets. Since most people carry awful advice, this is extremely unhelpful in practice.
* Some people have a crab bucket mentality towards others losing weight, and almost go out of their way to sabotage.