The problem is that people want to eat and they end up wanting more than they need. Controlling these urges is sometimes difficult. Especially if meals are high in sugar content. But we also like sugar.
The problem really is the curbing of desire.
The problem is not that we "want" to eat, in some sort of non-scientifically-deducible free-willed manner, but that the reference value for our "hunger" control is set too high, and so we feel hungry when we shouldn't, and don't feel full when we should. Interestingly, this (and other control systems) are all linked to serotonin: depression moves the reference value down, making you eat more to feel full; anti-depressants move it up, causing a symptom of "lack of appetite."
The true diet pill would just be a very selective SSRI, to move your hunger referent to the correct position without affecting your other controls. People don't the idea that they should be taking pills to solve their weight-gain because they have a "brain imbalance", though, so they will just keep trying, and failing, to use conscious willpower to overcome their controls, no matter what gets invented in the meantime.
The blood sugar/insulin "roller coaster" can be avoided: rises in blood sugar stimulate an insulin response, insulin is released to move the glucose, but eventually this causes a dip which triggers (somewhat misleading) hunger pangs. The trick is to get this to happen as little/gently as possible and those pangs go away.
I do it every day and I've lost weight for the past ~20 weeks without being hungry. No willpower is required, just consistency (and taking a great interest in it all...).