I always wondered about this - this was a fascinating read and it feels like a pretty complete list of possible factors. https://randomwire.com/why-japanese-web-design-is-so-different/ …
If Rakuten is one's understanding of the center of Japanese websites it feels relevant that Rakuten is really similar to an Amazon or Yahoo in terms of business model. Is its web design wholly unrecognizable next to either of them? No, doesn't really feel that way.