aggle-rithm
Ardent Formulist
It isn't. If species didn't evolve they couldn't adapt to changing conditions and would indeed die out. There'd be nothing alive left on Earth by now.
A lot of people don't realize that until complex, multi-celled organisms developed the ability to evolve (came up with a robust enough design that variation would not be fatal), there were no complex, multi-celled organisms. That's why most of the history of life on earth, by far, consists only of single-celled organisms.
It's already been pointed out, but the timescales involved defy imagination.