If people are actually powerless to combat a threat, it's rational to shut it out. There's no sense worrying about things you can't control. The irrational response is to obsess about it and take mental energy away from things you can control.

(This is also similar to why we don't worry about an asteroid striking the earth, a pandemic wiping out humanity, or nuclear war. There is little that we personally can do, so why spend mental energy?)