It's also partly because stories in which everyone is generally good people and nothing really bad happens are boring. Not just in the dramatic sense of conflicts in such settings involving such characters being trivial, easily resolved, and temporary, but also in the critical sense -- it's difficult to impart meaning or convey deep emotion with stories about good people being decent to each other in a peaceful, ordinary setting.
A fluffy slice-of-life story can be a good palate cleanser in the same way that a glass of milk or a little bit of ginger can, but would you want to live your whole life eating nothing but ginger and drinking nothing but milk? That's not to say that there aren't places for genuinely good individuals in fiction, anything but. However, that is always going to be in the context of a generally good character playing off of less than good characters and situations or vice-versa.
On a somewhat related note, "good" cannot exist in a vacuum. It's a meaningless idea on its own in the same way that "cold" is meaningless without heat. Furthermore, there's the issue of nuance: stories about perfectly good, normal people in completely ordinary situations with no real conflict are just as jarring and unrealistic as the polar opposite, because real people and situations are nuanced; to use the old color metaphor nothing and no one is perfectly black and white, nor are they "grey"; everyone and everything is a rainbow, a panoply of different and often contradictory motivations, causes, emotions, and goals.