When prices are high anyone who owns has incentive to prevent the supply problem from being fixed.
Unless… you have a land value tax
Their housing prices are being further depressed by the fact that they're now dying off fast enough that even Tokyo's population is starting to significantly decline. And that, in turn, is further compounded by a prevalent superstition in Japan against living in a house where somebody died, which helps to further reduce demand for many housing units that 'become available.'
Now it's rather the opposite. If you go to Japan on a Western salary (and especially after converting Western currency post ~2022), everything's dirt cheap for a foreigner, yet quite expensive for locals, which is much more akin to the economic state when visiting a developing country.
And so saying housing is cheap in Japan is kind of crass in a way. I mean yeah obviously it is, so long as you don't happen to live and work there.
Housing in Japan is kept sensible in large part thanks to their land value tax for real estate.