> All of that said, when we accurately track the amount of food people are eating, their weight does pretty predictably move with the calories in almost all circumstances.

This is only true in the short term - days,weeks. What is observed in the long term is that people, even the overweight and obese, reach stable weights without changing their lifestyle. So, if I were to start eating a surplus of 1000Cal/day compared to my current diet, and without changing my lifestyle in any other way, i would gain weight quickly until reaching some other significantly higher weight level, and then plateau there until I change other aspects of my lifestyle.

This may be due in part to simple energy calculations (I need more energy to move around a higher mass), but is likely also affected by other metabolic factors as well (e.g. I would also develop stronger muscles to hold my bigger weight, and stronger muscles directly need more calories to be maintained).

It's also extremely important, when discussing practical weight loss, to remember that appetite is also directly controlled by your body chemistry, and may vary wildly between people, probably explaining a huge amount of the observed weight differences.