Asklemmy

Post-war housing shortage led to tons of homes being slapped up quickly to deal with civilian homeless, returning soldiers/prisoners, and US troops. There were basically no standards (there were an absurd amount of homeless due to the firebombings and such) or building codes. Codes got stronger, but many houses were still poor quality (and burnt down often). Bonus fact: this is one reason there are such monocultures of certain trees in Japan; clear-cut to provide lumber and replanted with fast-growing trees for more lumber.

Secondarily, families were (and many still are) multi-generational. So, when a parent retires, the next generation will often take over and often knock down and rebuild. That's becoming less of thing, but it still exists.

Finally, especially when there were few/no regulations, things burnt down a lot, particularly during winter (heating) and when an earthquake would strike. Better standards now, but this was true for quite a while.

So, generally, treat any home/condo you buy as a depreciating asset. The land may increase in value depending upon where it is. In rare cases, the building may as well, but I wouldn't count on it.

That's at least a very TL;DR version based on what I know. There may be more that I don't know as well.