I've heard the opposite. Think of all the LED and neon lights and the HUGE amount of advertisements in, say, Tokyo. It looks kinda like their websites imo.
For a lot of people, there isn't a good enough answer yet and that is why the question gets asked over and over again.
I think it's because a website isn't a "downtown." It's more like a single building rather than an environment built by thousands of different people. But where is the Toyo Ito of Japanese website designers?
It's true those may be reasons why they look like they do, or part of the reason, but I think it's a question that is interesting. Even if Japanese websites get their "language" from those things, it at least seems like they should get their language from other places also, such as modern architecture or industrial design.
> such as modern architecture or industrial design.
Except for some landmarks, temples and gardens, contemporary architecture in Japan is surprisingly ugly and noisy.
I think most of the big sinners of Japanese webdesign are stores (at least the examples I can think of) and something like Bic Camera is more noisy and flashy in real life than their web presence.
If you look at landscape design, blank spaces in modern Japanese architecture, minimalist poetry, painting, pottery, film, the websites stand out a lot. There is no "zen" in Japanese websites (again, generally speaking). The "mystic" is how we perceive their work. It exists because it's just hard to quantify art and design. We know something's there though. Famous designers and architects like Frank Lloyd Wright go to Japan and are completely floored. Did many other isolated cultures' aesthetics regularly floor their visitors? People don't get that from Japanese websites... we perceive them differently, without any sense of the "mystic." And that seems odd.
I think it's an interesting question (kind of like "Why is the USA bad at soccer?" which I've heard being discussed a lot before.) but only because of Japan's unquantifiable success in other areas of design.