Because they can.
Think about it... hypothetically, what's to prevent Harvard from charging 50K per semester for tuition (not even for the year)? Nothing. If Harvard does that, nothing will prevent applicants from applying, because it's Harvard. Following suit, what then prevents Hopkins, Washington, etc. from doing the same? Nothing, because they're Hopkins, Washington, etc. Going down the ladder, what would then prevent some school in Bumfcuk, Alabama or something from doing the same thing? Nothing, because it's still a US medical school and is world-renowned as such. People will still apply, people will still matriculate and people will pay whatever it costs to go to a US medical school.
Besides, it isn't like you wouldn't be able to afford to pay the loans back once you're a doctor. Law school is expensive too and so are other programs with lucrative futures.
They can charge this much and there really is nothing we can do. If you choose to give your seat up due to finances, someone else will jump right into it before your heart takes its next beat.
Bitching about it is like bitching about death and taxes.