The average last-gen "AAA" title ran with bad IQ at 1280x720 and 30 FPS (hopefully). Now, you want 1920x1080 at 60 FPS. That's 4.5 times as many rendered pixels. However, people also want better IQ, better graphical effects, better lighting, and more detailed assets in their "next gen" games. But if you make each pixel just 4 times more expensive to render (which isn't really all that much) the new consoles would already have to be 18 times more powerful than the previous ones to maintain 1080p60. They aren't, not by a long shot.