This is the kicker. There are incredibly difficult to get, they are incredibly stressful, and it's another 3-5 years of your life.

If you allowed any non-us developers to practice in the US but you made them work a 5 year 80hr/week internship for 1/6th of what professional developers made first, that basically bans non-us developers from writing code in the U.S.

In brief, they have to sit the MLE (medical licensing exams), but the real hurdle is getting into a US-based residency program beforehand. In practice, this means only the best candidates tend to make it.