Why are all the prophecies about doom and gloom and none of them for good things to happen

It is because the odds of us avoiding catastrophe have been slim for a very long time. You don't need psychic powers to see that consumption, pollution, war, overpopulation, and human nature are eventually going to get us.
It is logical and is the expected result of our actions for us to crash and burn because of what we are doing.

Because doom & gloom has an end and good things portend the misery going on & on like the energizer bunny.