Every home I have visited in Japan over the past two decades has had A/C units. You may be right that in regions where it is very cold an A/C unit may not be necessary. However, as heat pumps are one of if not the most energy efficient ways to not only cool but also heat a home, I wouldn’t be surprised to find them even in colder regions.

Add on the fact that houses in Japan are generally considered a depreciating asset, so until recently demolitions and new construction were favored over renovations, I stand by my first statement.

Additionally, every place I have ever rented has come with at least one A/C unit, and additional holes are predrilled in other rooms so that split systems can be installed.