The realisation that any sort of threat from collision with cosmic debris exists is a relatively recent phenomenon. As mentioned above, it was only in the latter half of the twentieth century that it was generally accepted that the craters on the moon were caused by impacts as opposed to being volcanic in origin.
Major impacts are rare, therefore easy to dismiss as irrelevant to the current generation.
The destruction wrought by a major impact will be orders of magnitude greater than any that resulting from other natural phenomena. It is now technically possible to avoid or, at least mitigate the effects of impacts. It is now becoming clear that cosmic impacts have played an important role in the geological and environmental development of the Earth, and may even have been the dominant factor in the evolution of life. But there is a great danger in having minds so narrow that they can see no further than half a lifetime in each direction. The asteroidal message that matters most is that the past, and the future, may be hugely different from the present.