It's because the policy is to make admissions very strict, such that anyone who gets in is deemed someone who will complete the training eventually. So they reject as many as they accept, and then 95% of those who get in end up becoming doctors -- attrition is pretty negligible. By contrast other places (eg caribbean, overseas) take a larger percentage of people at the admissions stage, but weed out people who don't perform. The US view is that this latter approach is meaner, and given the expenses involved, it's nicer to never let someone start than take their money and fail them out. So that's just our system. That being said, if you end up repeating a year, which costs you 30k, plus basically eliminates you from the more competitive residencies, that's a pretty harsh result. So don't look at failing and retaking as a great option. But yes, if you get into a US med school you will probably end up a doctor if you are willing to put in the effort however long it takes.