Because that bores us to tears. Why sit on one's ass watching sappy shows only promoting good things, or severely downplaying human depravity, instead of actually going out and living a life well lived?

Not even in ancient Greece did decent folks go to the plays just to see their decency validated and mirrored. They went for the macabre, the tragic, the comic, the tragicomic, and the portrayals of evil and vice. As a catharsis perhaps, or somber warnings of how not to be.