Nutrition is the most important aspect here. Variety is not a nutrient and comparing fish nutrition to the lack thereof in humans is not going to lead to you feeding the best diet.
As far as fish wanting other foods over NLS, animals need certain nutrients and many are hard to find in nature. Therefore they have developed a high drive for certain things like fat. That is why we like things like fried anything or ice cream, in our past fat was hard to find and we developed a huge drive to eat it when it is available. Now we can find it anywhere (hard not to find it) and that is why we are far from as healthy as we could be. My guess is the other foods have higher levels of something (or many things) that the fish crave. They may prefer it more today, but long term it is probably an unbalanced diet and will lead to the usual cause of death for most aquarium fish, disease form years of less than ideal nutrition.
