...simply because critters do not EVOLVE! They mate and have offspring within there "kind" but that's it! Extinction, with the exception of dinosaurs, is due to man's selfishness and greed and inability to care for the earth's environment! Agree?
I'm pretty sure your "exception" was not a unique event, and in fact according to geological evidence, a lot more living things went extinct virtually at the same time as the dinosaurs, and that event is only one of several known "bottlenect" events in earth's living history.
Although invoking a further thought might seem too improbable for "science" to take it seriously, I would consider it likely, that there are similar "bottlenecks" in the grander universe, on uncounted planets now existing. . . . as well as in an endless cycle of "universes", and maybe in more dimensions that we are aware of. . . . .
with statistics, even "zero probabilities" do happen given infinite time and expanse. . . .
the unique thing about "life" is "consciousness", the ability to perceive something of a context or circumstance, and with it the "intelligence" to create a plan for dealing with it. . . . . on every imaginable scale of things. . . .
and that is why, even though the scoffers today snicker at "intelligent design", they will not carry the debate. The statistics say they are dead wrong.
the reason life goes on is because life has in it's nature the "intelligence" to take effective action to increase it's survival stats. We all do this individually, and so we are all living proofs of the theorem of "intelligent design", even the scientists who argue with such stunning stupidity against it.