Meteor or comet strike can easily wipe us all out just as it did Dinosaurs. And new science is suggesting impacts such as the one that killed off dinosaurs are more frequent than previously thought. So a big one (not necessarily as big as to completely end life on Earth but with potential to kills hundreds of millions if not billions of people and/or at the very least end our civilisation and set us back hundreds of years) could be happening every couple of tens of thousands of years instead of hundreds of millions of years as previously thought.

And smaller impacts that might not completely wipe us off but could cause millions of casualties are possibly even more frequent. Something like Tunguska meteor can potentially kill millions of people if it hits a big metropolis given our current population density. If it hits Tokyo or New York instead of middle of nowhere in Siberia, it would be devastating.