I suspect it is because a current PPL is required for international flights. If one doesn't have an active medical, then one doesn't have an active PPL. Within the USA the PPL holder without a current medical is allowed to fly under Sport Pilot rules only - but that's only domestically, since international flights are covered by the ICAO.

Sport Pilots (and those flying under SPL rules) don't meet ICAO requirements. Only one country (the Bahamas) has a bilateral agreement to recognize the SPL with the USA. It's something I was well aware of when I got my SPL - and didn't care about, since I don't anticipate flying to other nations.