Not everyone in Tokyo lives so close to train stations. There are places inside the 23 wards where the nearest train station is more than 3 kilometers away. You could walk, but it would take around 50 minutes.
I should also add that if you want to move around the west region of Tokyo, you're better off using a bus because there are no trains there that lets you move north or south. The Tobu Tojo Line, Seibu Ikebukuro Line, and the JR Chuo Line all move in the east/west direction.
