Another unique quality of humans.
Only humans can alter the entire atmosphere and climate, fouling the whole planet and rendering it uninhabitable for everything more complicated than a cockroach and much that is less complicated.
So, the ability to possibly detect some off-chance of a maybe asteroid and take a chance on deflecting it is balanced by the clear and present 100% chance of Man fouling everything's nest long before that asteroid might come along.
In other words, Man can, will and is doing in real time what some asteroid or comet might do in a million or 10 million years.
Bad trade-off.
Arf