Tim Noah’s answer:

Because it creates alienation...For a long time economists said: Wait until productivity rebounds. Then working families will get their share. But when productivity rebounded like crazy in the aughts, working families saw no reward. What this means is that if you’re at the median you have no positive reason to care how the economy does. Your only motivation is fear—if the economy does really badly you may lose your job. But there’s no upside.

That’s from Noah’s ongoing Slate chat with Matt Yglesias. Noah’s new book on inequality is here.