I've lost 90 lbs, achieved a healthy body weight, and kept it off for years, but I keep trying to lose just 10 more pounds. I only make progress when I'm religious about calorie counting, including weighing every meal or needing to do precise food prep beforehand.
That's such a pain in the ass that I inevitably fail and keep returning to my body's new average weight.
I think you're right about truly obese people. They're eating so many calories that they would lose weight if they could manage a better diet.
For people who aren't obese, the same principle may apply, but it's now a matter of much tighter margins and much greater precision, and it's remarkably difficult.
Natural bodybuilders seem to manage their weight with precision, so it's not impossible with knowledge and effort. But I suspect very few people are capable of managing that if they tend towards obesity to begin with.