So said Larry Niven, but it isn't true. Dinosaurs became extinct because they didn't have telescopes and grain silos.
Diverting asteroids, particular the Chicxulub size objects big enough to wipe out dinosaurs, is a hard problem. Fortunately, you get most of the benefit of diverting a killer asteroid just by identifying it in advance. Advance warning allows you to evacuate the threatened area, and people are worth more than buildings. Even a year of darkened skies is survivable if you stockpile food in advance.
Detecting the potentially dangerous asteroids is the low-hanging fruit.