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.
I think that's more perception then reality. I live in a 30 year old house and it could easily last 100 years if properly maintained, but there is little incentive to improve the building because it will not improve the resale value.
Living in a city which city center was (re-) build between the 16th and 17th century, the 30 years till worthless puzzles me.
Even the house I live in is over 100 years old and in excellent condition.
Why do houses loose their value so rapidly in Tokyo?