When they view these planets from a telescope they are not seeing the planets as they are now but as they were in the past due to the time it takes for light to travel to our eyes. Kebler-bb is 600 light years away, so when they see that planet in the telescope they are seeing it as it was 600 years ago.

Even if they were to discover that life exist on the planet, they can't be certain that it still does. Something could have happened that they aren't able to see yet that killed all the life there.