Late comment, but I have a relative in Japan and I asked about this:
"The empty houses are caused by two factors. Aging population with younger people moving to the cities and the the way property taxes work. Empty lots have much higher taxes than ones with houses on them. So if a house becomes empty because of people dying off, it’s cheaper to just let an old house be abandoned and wait until you are ready to rebuild than to tear it down and pay high taxes. Houses aren’t worth much in Japan. Average houses aren’t built to last as earthquakes and humidity tend to cause deterioration. However the government will take the property if the taxes aren’t paid on it for a certain period. The free houses isn’t really a thing."
"The empty houses are caused by two factors. Aging population with younger people moving to the cities and the the way property taxes work. Empty lots have much higher taxes than ones with houses on them. So if a house becomes empty because of people dying off, it’s cheaper to just let an old house be abandoned and wait until you are ready to rebuild than to tear it down and pay high taxes. Houses aren’t worth much in Japan. Average houses aren’t built to last as earthquakes and humidity tend to cause deterioration. However the government will take the property if the taxes aren’t paid on it for a certain period. The free houses isn’t really a thing."