The following is probably wrong in detail.
In Australia, after graduation, doctors are required to work for a period as interns in public hospitals. This is a period of working while half dead. They routinely work thirty hours without a break, and eighty hours in a week.
While everybody knows they make mistakes, no-one wants a scientific study.
I recall reading/hearing that this practice exists as a stress test for prospective doctors. How well do they react when working on very little sleep? They may be involved in a major emergency someday -- a natural disaster, for example -- where they will have to work on very little sleep for extended periods. Arguably, it's best to uncover their weaknesses now rather than later. -- BrentNewhall
Interesting theory. The same practice is applied in the UK, I don't know of any justification for it beyond avoiding the need to train and pay two or three times as many doctors. Note that in this case, the term "death march" is not figurative. People die from this. Patients mostly, but also doctors. That seems a steep price to pay for stress-testing trainees, don't you think?
I'm not defending the practice, but on the other hand, death happens often in hospitals. Perhaps this is justified by saying, "It's better to have a few casualties now as a result of Death Marches than have a lot of them later as a result of hiring a doctor who's routinely unable to operate on reduced sleep." -- BrentNewhall
A combination of institutional inertia and financial savings are also responsible for (at least) the UK's situation. As a House Officer, your future career is in the hands of senior consultants, who, because they went through 90-hour weeks to qualify, see no reason that you shouldn't too. Also, a doctor is paid less per hour for overtime (or at least used to be) than for normal work, making it cheaper to make people work overtime than to rationalise shifts.
The test argument is only correct if the test can be failed; in the UK, there is such a shortage of doctors that the NHS simply can't afford not to hire qualified people.
The main reason why junior doctors work are worked so hard and most do not complain is career protection (the closed shop): it ensures that the doctor career pyramid is not that much of a pyramid, that is that many more junior doctors eventually become consultants than it would be otherwise the case.
If there were 2 or 3 times more interns (that is, if the doctor's guild permitted 2 or 3 times more places at medical schools), only 1/2 or 1/3 of the resulting greatly expanded number of junior doctors would be able to reach the top of the profession.
Most understand this and willingly put up with extreme overwork at the beginning of their career to have a much higher chance of high earnings later. Patient safety may not be that high a concern compared to career protection.
-- Blissex
I think it's because WeveAlwaysDoneItThisWay?.
"I recall reading/hearing that this practice exists as a stress test for prospective doctors" This seems to me like encouraging "driving under the influence of alcohol" to weed out those who are less tolerant to alcohol. Surely, our roads would be so much safer if those who can't drink and drive kill themselves off in road accidents.
See also: DeathMarch, SleepToWork