There are two reasons for this situation in USA. Firstly, doctors make so good money that sometimes they cannot say No.

Long shifts are generally a resident doctor problem. Resident doctors probably earn less per hour than you.

Second reason is an inadvertent continuation: US hospitals cannot afford to employ sufficient number of doctors so they rely on residents (physicians-on-training) so much that they force them to work non-stop.

It is true that training hospitals may not have resources to fund a sensible roster, even if enough qualified candidates are available.

It is also true that medicine is a fascinating undertaking and some residents are reluctant to clock-watch while care is underway.

Some doctors are even altruistic. ;-)

The latest trend is to hire "nocturnist" physicians who work only at night and have no patients of their own. Obviously the cost can be expected to rise.