Because eventually, there will not be many jobs left that cannot be automated. Cleaning and some of the trades will remain as long as we get iRobot-style droids with dexterity and intelligence to match a human, as will politics and management, but everything else will get automated.

The question is, will we manage to shift our society away from the millennia-old model of "one's employment defines one's worth in society" to "everyone is worth the same in society" in time, or will there be bloody fights in the transition?