If you were to do some actual calculations of the type of detection and analysis capability, and the power required to effect a lessening of a comet or meteor event, and then the cost-effectiveness to construct and maintain a "planetary defense" against such a rare event, you might not make such a sweeping comment. This problem is not trivial, and the cost of acquiring and maintaining such a system right now is simply out of reach. I wish that were not the case, but that is the truth.
The energy of a such a mass rushing toward Earth in some such event is equivalent to billions or even trillions of H-bombs. It might well require the power of thousands of such engines to sufficiently deflect such a moving body. NASA gets flak for contemplating the launch of extremely small reactors for powering experiments, yet you wish to propose launching the equivalent of hundreds or many thousands of H-Bombs with some of the most powerful and untested rocketry, and maintaining that in pristine condition for hundreds of years or even millenia?!
God/Nature is much more powerful than most humans imagine.
Even a simple thunderstorm moves millions of tons of mass... we, though we are so proud of our "high-technology", are not even in the same league.
The real problem with a mass rushing toward Earth is that it is unguided. We do not need to deflect or stop such a body; we only need to alter its course to a slight degree.
Like cancer, early detection for this problem is needed. Once an oncoming body is detected and its course calculated, it should be relatively simple to also calculate the optimum course correction which would lead it to a better location.
One might even hope it could be used to further other goals of future space activity.
Anyway, as to moving it, even the slight amount necessary, there is a procedure which can be guaranteed to work, if given sufficient resources and time in which to act.
That is the landing on the oncoming body of a mass-driver. Whether using solar power or nuclear power, the driver would steadily dismantle the object, flinging the small portions away in a direction calculated to achieve the course correction desired.
If need be, swarms of such devices could be employed.
Since we would be using the asteroid's own mass as a reaction fuel, it would be impossible to run out. The only thing we could run out of is time.
It is for this reason that many who are concerned would like to see additional resources made available to search the skies, and to prepare the simple machines that could prevent our sharing the dinosaurs' fate.