Because many of them were built cheaply to begin with and develop problems over the decades. They weren't built for long-term living, and in a sense it was the right move since Japan was developing rapidly up till the end of the '80s. (Well-built exceptions probably exist, of course.) And then there's increased earthquake/disaster risk with poorer construction.

It's something I wondered about too until I looked at descriptions and photos of >30-year-old houses on real estate sites.