But the last period of the Cretaceous was 9 million years long, so we are only saying that the extinction occurred somewhere in that 9 million years. It isn't always visible for a start. We can only say that the species was not extinct at the level where we see the most recent fossil individual. The Hurricane of 1989 in Southern Britain made all the papers, as do others in the Caribbean and North America. We make differences appear in a continuum by defining arbitrary boundaries.
Mass extinctions are a bit like that too.
There are some mass extinctions, but usually there are not many extinctions going on. So mass extinctions are there, infrequently, as part of the normal pattern of things - small events are common, large ones are rare, like hurricanes. So big extinctions happen sometimes. The less random it is the more effect it will have on evolution. The Spaniards introduced horses to the New World in the sixteenth century, with a devastating effect on the natives.