People do, practically by construction of the term. Rationality is the social norm. Irrationality is typically defined as some variant of mental disorder.

Rational patterns of behavior are critical to an individual's health and well-being. They're also patterns that can be observed, extrapolated, and exploited. So you'll often see an individualistic rational action (for instance, fleeing from danger) manipulated by a malicious actor (for instance, someone shouting "Fire!" in a crowded theater) in order to achieve a large scale negative result (a stampede or crowd crush).

You can point to a crowd crush and conclude "Everyone in that crowd was irrational" when no single individual did anything at any given moment that didn't make sense from their point of view.