Judging from the small amount of Japanese cities I've seen, it seems that there are nearly no pre-WWII buildings left. Of course a lot of this is attributable to bombing during the war, but I would think there should still be more than there is.
It really depends on what types of buildings you're looking for, and which city you're in. Many cities got burned, sure, but many places were either not very strategically important or specifically spared (Kyoto comes to mind). Traditional buildings like bathhouses and hot spring installations are largely intact (though many were built after WW2 for tourists), but only a handful of castles still exists from before WW2 (most we see today are rebuilt). The reason for this is because centralization efforts during the Meiji Restoration required castles to be torn down in order to prevent regionalism that was rampant during the Edo period and feudal Japan, and the rest suffered from the war and natural disasters.
If we're talking about normal everyday buildings, then it's mostly modernization - tearing down old buildings to make way for high-rises and the like - a natural process that happens the world over.
adding to /u/_dk's answer
It's good for people to keep in mind that while a 100 year old building is cool and full of heritage and history, a 20 year old building is old and drafty and hard to keep clean. What I mean by this is that it's not the destruction of ancient buildings that is usually the problem, but the normal everyday renovations and upgrading that happens naturally in a city over a lifetime.
We see the same thing happening today of course. In Beijing many people are lamenting the loss of the older hutong homes, but these aren't the same people that have to live in those conditions. For many of the local residents, a warm place in winter is more important than preserving their 20 year old home.
That said, there are still a large number of Japanese buildings still intact in Taiwan, where the love of French Art Deco interpretations was alive and well in the Japanese Empire, but also more traditional building methods. While today many of these are becoming historic tour spots where you can go learn about the former resident's life, a lot are still just sitting empty and in a state of disrepair, having been bought out by local governments that lack the funds to do the repairs. Do a Google Image search for "Japanese homes in Taiwan" and you'll see many photos of both types. Taiwan was also heavily bombed by the US during the war and so, like Japan, rebuilding was necessary.
Judging from the small amount of Japanese cities I've seen…
You also need to get out of the city centres where the greatest amount of gentrification is likely to have occurred. Kansai has a number of smaller towns that still have a large number of these homes, while nearby downtown Ōsaka does not. If you'd like to see what some of these look like you can check out some of the real estate offerings in rural Japan. That page is the category specifically for this type of home (古民家) which get listed in the "rural living real estate" (田舎暮らし物件不動産) sections. Renovating them is kinda a thing and there are a number of Japanese television programs specifically focused on this, using traditional craftsmanship methods to renovate homes in disrepair for modern families. It's my guilty television pleasure.
I'm actually covering this in my current master's thesis! Unfortunately this bit only exists in note/initial draft form and I'm not going to copy stuff directly as it hasn't been through turnitin yet and a plagiarism accusation (if this is for a paper you're writing) isn't worth the karma, so I'll hastily re-write!
This covers your question from a more philosophical perspective and not from, for example looking at Japanese planning policy.
The reason while possibly due to the extensive bombing campaign (I’m yet to explore this, so far all I have is France losing 15% of its heritage buildings during WW2), is also due to different cultural practices and beliefs in built heritage between the West and East, with conservation being an idea stemming from the Enlightenment era in Europe.
Built heritage conservation is managed on an international scale through various treaties and charters governed by UNESCO, arguably the most important of these is the Venice Charter (1964). While it was envisaged to set out an international framework for conservation efforts, it has been criticised frequently for being a Eurocentric document and is only relevant in parts to East Asian architecture. Effectively, the charter is written with so called ‘intended monuments’ in mind; think Cathedrals, Palaces, Centres of Government (e.g. Palace of Westminster). The Venice Charter struggles, however, with heritage not built on ideas of grandeur, instead of spiritual and intrinsic values (such as tradition and ceremony) [1]. It is argued that if you look at the East Asian civilisations, barring the odd exception such as the Great Wall of China, despite their extensive histories back to the (European) classical era there is nothing of a similar architectural scale to say the Great Pyramids at Giza [2]
The reason for this is that the focus of East Asian architecture is not on the physical details, but instead on the setting of the building; looking at its setting and how it interacts the natural surroundings. This also coincides with massively different understandings and practices within maintaining the concept of ‘authenticity’. In Japan, traditional architecture is a timber frame, given prevailing climatic conditions (tropical) decay and loss of this timber over time is recognised as inevitable. There is no literal translation of ‘authenticity’ in Japanese and no direct equivalent, in fact it is used without translation mid-sentence arguably limiting understanding. In Korea, the onus is more on so-called; ‘felt histories’, here authentic and replica materials are used together without hesitation [3]. The effect of this is that a temple can be (and often) is rebuilt – the value is not in the ‘bricks and mortar’ but instead lies in the intangible.
Note, there have been charters since the criticised Venice Charter which attempt to address the Eurocentricism such as the Nara Document on Authenticity (1994) which adds UNESCO recognition for intangible values such as spiritual ones.
It is a problem of politics and law.
Very difficult it is to remain the old building in relation to seismic reinforcement in Japan.
Great cost in order to pass the old building to the law of the seismic reinforcement is applied.
And was it difficult to leave the old building is the Liberal Democratic Party and construction suppliers.
The problem is the urban re-development, which is also where the country.
It is not about the old building, recently they are trying to destroy the important ancient ruin for build a bypass road.
http://www.chunichi.co.jp/article/shizuoka/20150701/CK2015070102000104.html
(article in Japanese)
