Why do patients lie to their physician(s)

My girlfriend is a doctor.

She occasionally expresses her amazement at the frequency with which patients lie or withhold information that is essential to proper diagnosis and treatment planning, thus deliberately putting themselves at risk.

She wonders if it is an “American thing”. (She is from the Middle East. “Her people” she says “would not be so foolish”. As a rule, I avoid cultural debates with her so I do not know if that part is true.)

But still, why would somebody lie to a physician about their history, drug use, medications, etc. and put themselves at risk.