Actually, they can.
The US isn’t in Japan because we want to protect the Japanese (at least primarily, anyways), it’s because it’s in our strategic interest to be there both as a counter to the ChiComs AND as a means of restraining the Japanese from their barely-deniable desire to remilitarize in the face of the ChiCom threat.
The truth is that while most of Asia fears the ChiComs, they also HATE the Japanese. Their cultures have very long memories, and don’t look back too fondly on things like the Rape of Nanking, the Asian occupation, Unit 731 and Comfort Women.
The US leaving the area would be like an adult leaving a room full of aggressive, angry and competitive pre-teens, who don’t get along and are all armed with high-powered firearms.
The Japanese government pays for 100 percent of the cost of the American bases and personnel in Japan.
Might as well build their own...