If you think about that, it makes sense. An applicant for a U.S. certificate based on a foreign pilot certificate must hold a valid U.S. medical certificate OR a valid foreign medical certificate. If a foreign medical certificate is used, it must also include an English translation or be accompanied by an English translation signed by an official of the foreign aviation authority that issued it. Some governments enter periodic medical endorsements on a pilot license that effect its currency. Therefore, if the foreign pilot license must have a medical endorsement to make it current, an FAA medical certificate alone will not satisfy the regulations.
Some countries stamp their pilot certificates to show continued medical qualifications.
Without a current medical qualification stamp the foreign pilot certificate is invalid and may not be used to obtain a U.S. Private Pilot Certificate under FAR 61.75 . In this case simply obtaining a U.S. medical certificate does not make the home pilot certificate valid. Your home pilot certificate must be valid and can be used to act as a pilot in command of an aircraft in your home country. In essence the pilot certificate is no longer valid until medical qualification is obtained. Therefore, in this situation, a foreign pilot needs to be prepared to prove his foreign medical qualification standards and its effect on the validity of the foreign pilot certificate held.