If we as human beings ate only when we were hungry and stopped when we were full, the diet industry would not nearly be as huge as it is today. But, it is not that simple. People eat when they are tired, lonely, and sad or feeling isolated. Sometimes people eat when they are joyous or really happy. In either case, appetite suppressants would not be helpful since people are not eating to satisfy hunger.
Human beings are the only species in the universe that have such a problem with obesity.
This is probably due to our over abundance of food sources and our tendency to link other emotions with feelings of hunger and food. Every other species of animal eats for survival and no other reason. When a cheetah feels hungry, she hunts for food until she finds it and then eats to fullness. But, people are different and this is why appetite suppressants might not work for some individuals because they are not eating to satisfy their appetites, but eating for other reasons. For example, people eat popcorn at the movie theater out of habit, not because they are internally hungry.