People aren't so much scared of what could happen as they are about inability to do anything about it.
Being careful around traffic makes getting hit by a big truck less likely. Diet and exercise are not panaceas but are mitigating factors for lots of medical conditions even if you're genetically or epigenetically predisposed to them. Modern medicine, although imperfect, gives us far greater control over both of those types of things even after the fact.
In a plane crash, unless you're the pilot or mechanic, there's not much you can do. In an asteroid strike, unless it's a smaller one we know is coming and can evacuate people, there's not much anyone can do. With a disease with a high infection rate, high mortality rate and no known effective treatment, such as Ebola until recently was, there's not much that can be done other than avoiding high-risk areas and quarantining people in those high-risk areas.
Regardless of the odds, people fear loss of control sometimes as much or more than the actual negative impacts. Some competitive team sports athletes have a more difficult time watching their team from the sidelines after an injury than they have emotional difficulty in their recovery.