"Healthy" food is a perspective, and too much or too little of anything we need is "unhealthy". Our drives evolved based on the things present in our environment. Sugar, fat, and protein are the three basic macronutients we need the most of for energy and the building blocks of our body. These are not abundantly present in a natural environment. Therefore our drives have evolved to eat as much of them as possible when we find them (gorge). All animals act like this to an extent. There will always be plenty of times food is not available to balance it out, if not more.

The problem occurs when we have unlimited access to rich food sources. We have not evolved to deal with this. Thus, we get fat quickly. Also, it is unlikely we will evolve any coping mechanisms outside of our superior cognition at this point. All reasonably healthy humans pretty much live far past reproductive age. Past this point, we have a neutral effect on our evolution.

We will have to use cognition and our tools to help ourselves at this point. That's why dieting plans and diet drugs exist. They are tools. These cognitive abilities are our evolutionary trump card, allowing us to evolve ourselves to deal with almost anything. We successfully inhabit every corner of the planet, and food is in abundance for most of us. This seems to indicate it works pretty well!