Obviously eating less will cause weight loss. The problem is actually being able to eat less and to keep doing that for life. Maybe that is not a problem for you ... so far, but it is a real problem for many people.
This article could shed more light on the obesity problem and solutions to it. For example, it could very well be that our intestinal flora influences our own decision making. There is at least one known case of a micro-organism that does: the toxoplasma gondii.
It is also apparent that people become addict to sugar and that definitely would influence decision making.
On top of that, our behavior is in great extent influenced, if not determined, by the chemistry of our bodies. It may very well be that chemical imbalances caused by a combination of diet, intestinal flora and hormones, causes obesity.
In short, saying that it is simply a problem of calories in and calories out is a gross over-simplification of the problem. Science is way beyond that point.
I don't know if you meant something else here, but of course eating less causes weight loss.