Because not all of us are so selfishly narcissistic and are rather able to consider the effect upon the lives, extant and not yet born, of our current decisions. Your apparent lack of empathy, and similarly our cultural worship of selfishness and self-deception, are foul clues as to our eventual doom. If choices today are critical in determining the viability of human life over the next several decades, but those choices conflict with the selfish wants of children in the present, and those present-day children choose to disregard future generations, then what hope do we have?

The Great Filter for our species is capitalism.