Where do you think we get the catastrophe movies? Based on a true story. You guys think the world can get destroyed by a large asteroid, but can't believe a world calamity has happened already.
The catastrophism vs uniformitarian debate. Both are likely true - uniform rate change between catastrophic events. Of course, relatively uniform rates differ in different environments.
Your point is well taken - many scientists believe in comet and asteroid extinction events but deny the possibility that earth's accretion of water can in some cases have been catastrophic (as in the Noachian flood) rather than always uniform. I think the reason is bias.