Can't forget metabolic disorders too. I'm a T2 diabetic. Well controlled, to the point I've had a few doctors literally not believe I was diabetic (at the time my A1c was like 5.1, a decade later it's generally between 5.5 and 6). I'm pretty strict with my diet. I occasionally will fall into snacking, but when that happens, I literally just stop buying snacks altogether to prevent it.

I eat the same meals often, and roughly track my calories. I do intense workouts weekly, and roughly tracking via my fitness watch indicates that I'm burning around 800kcals per HIIT cardio workout 2-3x per week most weeks, plus my other sessions with weight training. I'm taking in probably 1800 calories give or take most days, and burning off over an entire extra day and a half worth of calories weekly. But I'm still struggling to lose weight. Even at what should be a high caloric deficit where I'm feeling regularly straight up ravenous, it took me a year to lose around 15 lbs, and my weight still flip flops up and down weekly, even though the overall trend is downward.

I'm a big believer in fitness, in the idea of calories in -> calories out, and managing my TDEE. But it's just downright harder for some folks, and I'm apparently one of them.