That’s because housing is so expensive in Japan that multi-generational mortgages must be taken out to buy a house.
Basically, whomever wants the house is going to have to take the parents.
Also in Korea (everyone wants to live in Seoul) and Hong Kong (really crowded), Tapei might be too.
Even going back to ancient times it was common for the patriarch to keep his thumb on the kids and grandkids. Today many live together but mostly in K-drama’s do they control the lives of their adult kids.