Maybe the trouble is people exercise and then they think they're hungrier than they are so they eat more calories than they even burned.
Then there's the soft factors. Counting calories is a lot of work, not many people stick through it. You end up with a lot of gaps in your data anyway, unless you give up eating foods that you can't source good data for. Psychologically we are fairly bad at estimating portion sizes. There are a ton of situations (restaurants, potlucks, etc.) where calorie counting becomes impractical or cumbersome.
On top of that, there's the changes in mood that come with reduced caloric intake. Again, different from person to person. Not simple.
I guess if you phrase it as "eat less, lose weight" then it's simple, but eating and your waistline are far from the only thing in your life. Calories in, calories out is a pretty solid theory but doing the numbers is a different matter.
If I eat over 3k calories, I gain weight. If I eat less than 3k, I lose it.
Not tracking my calories and quitting my workouts due to wisdom teeth extraction and a nasty sinus infection turned chronic has cost me 20lbs.
20lbs I worked very, very hard to gain. Consuming half a gallon of whole milk a day, forcing myself to eat even when I had no appetite, etc. Working out. I felt great and looked better than I had in awhile. All gone now, back to square one.
My goal is to get over this sinus infection, (black plague, whatever it is) continue counting calories, as well as exercising, to put that weight back on and then some.
I must say that the bends in the graph is very noticeable.
If I let my guard down I slide back to 145 lbs within a few weeks and have to climb the mountain all over again. Age has yet to cure me of this.
We've long thought of weight loss in simple "calories in, calories out" terms. In a much-cited 1958 study, researcher Max Wishnofsky outlined a rule that many organizations — from the Mayo Clinic to Livestrong — still use to predict weight loss: A pound of human fat represents about 3,500 calories; therefore cutting 500 calories per day, through diet or physical activity, results in about a pound of weight loss per week. Similarly, adding 500 calories a day results in a weight gain of about the same.
Today, researchers view this rule as overly simplistic. They now think of human energy balance as "a dynamic and adaptable system," as one study describes. When you alter one component — cutting the number of calories you eat in a day to lose weight, doing more exercise than usual — this sets off a cascade of changes in the body that affect how many calories you use up, and in turn, your body weight.
At the end of the day, weight loss is still calories in, calories out -- as frustrating as that can be.