You judge how common something is by how easily examples come to mind
Tversky and Kahneman showed that people estimate risk and frequency using how readily examples pop into memory, not actual statistics. Plane crashes and shark attacks get heavy news coverage and feel vivid, so people routinely overestimate how often they happen compared to far more common but less memorable risks like heart disease or falls. The availability heuristic is fast and usually useful, but it distorts judgment whenever memorability and real frequency diverge.