One reason that so many people discount the risks of oil depletion and climate change is that their experience tells them to do so. We've had high oil prices and tight oil supplies before, and always new supplies and declining prices followed. For many we are just in another market cycle, and there's nothing to be worried about. And, when it comes to climate change, well, we've had hot summers before and even if the climate is warming a bit, we'll adapt.
Both these observations are subject to what's called the problem of induction. In a nutshell, we believe that because a certain event has reliably repeated itself in the past or because certain conditions have prevailed for a long time, we can always expect more of the same in the future. If that were true, there would come a point in our lives when we would never be surprised. But as it turns out, humans are continually surprised, which shows you that the problem of induction lives on.