Plus SO often implies OP was a citizen and then SO comes in on a priority VISA program. They do check to ensure you really are romantically involved. There are a number of poor who decide nearly any American is a better life than they could have at home and so will sign up for a blind marriage to anyone. A doctor likely has means to do some verification of a potential mate before committing, but if they find life bad enough back home they can have low standards. (from what I can tell it mostly works out for those doing this - some get abused, but most seem to have a good life)
Also, we are definitely missing a large part of the story. There are very few medical schools outside US that will qualify you to practice is the US. Getting recertified in the US is extremely difficult, especially from a developing country with much worse healthcare standards.