WOW a personal attack to start things off, nice. I'll take you up on your offer to point out the several reasons. Though I doubt you have any you will actually offer up.
It wasn't intended as a personal attack. I can see why it could be interpreted that way, and my apologies for wording it ambiguously.
But since you ask:
1) Too few calories = fairly immediate problems such as fainting, lethargy, inability to be productive, etc.
2) an improperly balanced diet could result in a myriad of health problems; look up deficiency symptoms for many vitamins (and frankly in terms of scoring I'd say the list of symptoms for each deficiency counts as a reason in itself)
3) Balanced diet again - calories are obviously not the be-all and end-all of diet planning, it's perfectly possible to have an utterly crap diet which is calorifically sound.
4) Saying that it's purely a matter of cals in < cals out is gross over-simplification; anyone who has attempted to lose or gain weight will attest to the fact that say simply under-eating or over-eating for one day will not result in noticeable (ie. with a set of weighing scales) change, because the body is capable of altering its requirements depending on the amount of food that's coming in. For example, starvation tactics are well-known to cause inconsistent outcomes.
5) Gross-oversimplification again - I have a diet plan that has worked in the past to reach my target weight. Due to extraordinary circumstances I went off the plan for a while and then went back on it, and obviously my weight drifted off course. Despite being back on it for at least two months, my weight has barely shifted (though in the last week it has started to shift again).
6) Saying it's only just a matter of cals in < cals out will make many people think they can 'bargain' with their body along the lines of "well, if I grossly under-eat one day, it will off-set the day that I over-ate and I will continue to lose weight": Nope.
7) Peoples' lack of understanding of dietetics leading to assumptions based on the incorrect things that they "know", such as assuming that a food substance prepared in one way will have the same amount of calories to if that food has been prepared in a different way.
So you really think that without a serious medical condition, someone who is eating less than the body needs to operate on a daily biases they will not lose weight?
The OP has a serious medical condition, which is why I'm trying doing my best to avoid giving advice that might run counter to his doctor's opinion, because I don't know him, I haven't seen his notes, and I'm not a medical professional. Have a browse through his posts if you don't believe me.
So wait, does this scientifically sound diet have more or less calories than the body needs?
Of course a scientifically sound diet (for someone who needs to lose weight) will have less calories in than for one for someone who is maintaining their weight. My original point was that you were grossly over-simplifying the solution by suggesting that that's all there is to it.
In the long term, the most important point by far is for a person to be able to come up with a diet plan that helps them maintain their weight and get a reasonable amount of exercise into the mix, because a diet plan that feels like self-punishment is not a sound long-term strategy for most people. The same applies at least to some extent with the diet a person adopts in order to invoke the weight change, because weight change is generally a slow process.