Because a white lab coat says "I am a scientific healer." The knee-length coat in medicine crossed over from the laboratory sciences at the turn of the 20th century. Before that time, medicine was generally seen as the haphazard province of quacks and frauds, and physicians wore street clothes even in the operating room. As the field developed into a respected branch of applied science in the early 1900s, doctors adopted the costume of the laboratory as a way of bolstering their scientific credibility.
In pre-white-coat times, physicians used primitive tools and techniques and had little formal training. (Medical school could be finished in a year.) Early doctors competed for legitimacy (and patients) with other healing arts like homeopathy and medical eclecticism. But the development of antiseptics and anesthesia, among other things, demonstrated the exceptional power of science to improve health. Doctors strove to become more scientific, in practice and in dress. The lab coat served both purposes by providing a (supposedly) sterile work environment and soothing patients with its air of scientific authority. The traditional lab coat was beige, but doctors adopted white because the color symbolizes life and purity. (In earlier times, doctors were more likely to wear black, in keeping with the high mortality rates seen at hospitals. The nuns who served as nurses often wore black habits.) By 1915, physicians working in hospitals had for the most part switched from street clothes to white coats and pants.
With their scientific bona fides firmly in place, doctors today are divided on the white-coat question. Supporters say the coat instills docs with a humbling sense of responsibility and puts patients at ease, while detractors see it as an alienating symbol of medical hubris. More than 100 medical schools host "white coat ceremonies" where first-year med students are outfitted with shortened versions of the white coat, and the coats are ubiquitous at large teaching hospitals where they help differentiate between doctors and students. However, doctors in smaller hospitals and private practice are more likely to wear regular clothes. A recent study suggests that only 1 in 8 doctors actually sport a white coat at work. Perhaps the most ardent supporters of the garment are patients: In one study, 56 percent of those surveyed believed doctors should wear coats, compared with only 24 percent of doctors. (Elderly people tend to be most supportive of the white coat.) Another study found that patients were much more likely to trust a doctor if they were wearing a white coat than if they were in scrubs.
If hospitals followed the AMA resolution and banned the white coat, what would doctors wear? The Scottish National Health Service outlawed white coats in 2008 and instituted a uniform of color-coded scrubs for all medical personnel. The Mayo Clinic doesn't allow white coats; their doctors wear business attire.