Authentic biology! The heat of spicy foods is intended to discourage animals from eating them. However, humans, being the cunning little bastards we are, found that spicy plants like peppers added flavor and kick to bland diets. This goes for everyone from the ancient Aztecs (who ate mostly beans, corn, and squash) to modern MREs (which are mainly composed of food-like products intended to maximize caloric and nutritional density at the expense of everything else).
Humans tend to make the same kind of mistakes. Like touching a red hot burner even though they have been told not to, among others. For a yukkuri however, one mistake can easily mean death.
Also, I don't know why people always dump on MREs; those things are good! Granted I haven't had to subsist on them totally, but they're good for camping and the like.
I think it's mostly a legacy of things like K-rations, which were mostly mealy and flavorless. It's also undeniable that nutrition is what they prioritize when making them, rather than flavor.