The US medical residency was mostly created by William Halsted, a very influential surgeon. He was partly responsible for this culture of sleep deprivation, insisting that residents be on-call 24/7.

He also was a serious cocaine addict who injected it every day. It's of course an oversimplified historical narrative, but at least part of the reason medicine does things this way is the traditions were set up by someone out of his mind on coke.