Because these problems for humans started existing with absolutely modest level of AI development and nothing remotely similar to super-intelligence. So the argument is - if we have serious problems now as small-level, narrow-focused AI "does not consider" some of the side-effects, imagine the problems if we reach high-level, narrow-focused AI...So author's argument is that we need to change the way we're building it so we either set the goals to more broadly ensure human benefits, or build something else other then the narrow-focused AI (which surely is another Pandora's box).