You have to know that Rakuten itself doesn't do much beside the frame. Rakuten is an Internet Mall - a place for merchants to show their products. These people are just sellers, not designers, and they are allowed to customize their products' and shops' pages however they like. Rakuten is to blame for not promoting consistent design among the merchants, but merchants themselves are to blame as well, for tons of noise in the pictures. When you compare, on Amazon pictures are big and clear, while on Rakuten they are there because seller just must put something. Often when you click the thumbnail, you get... the same thumbnail instead big product picture (which makes me mad.) And if sometimes 60-70% of picture space is covered with text. But what really makes me wonder is - how this strategy works? They are there, they are successfully selling their products! It's not only Rakuten, it is visible everywhere on the streets. Posters and commercials are full of noise. Not only text, mind it. It is common practice to have a person with tube in front of your store to praise the products they are selling. Noise is everywhere here. Everywhere except luxurious boutiques. :)

Rakuten doesn't look any more crowded than say, Amazon. It seems to be a shopping website, so perhaps the amount of information is more related to the type of website than the culture in this case.

If anything I think Rakuten and Yahoo.co.jp are the outliers. They look more or less the same as they did years ago. I think they stay the same because they don't want to upset longtime users. Maybe like how Craigslist keeps the same old school layout because people are familiar with it.

My argument is that, if all this stuff about Japanese people liking information density were so true, then why do new websites not look like Yahoo.co.jp? Nobody is making web portals anymore. Those sites are relics of the past.

Also, I can find plenty examples of modern webpages besides the two I linked. How about Mericari, who have taken over the second hand goods market from Yahoo Auctions (http://www.mercari.com)? How about Gunosy, a news aggregator who have no doubt stolen tons of traffic from Yahoo (https://gunosy.com)? How about the bank AEON (http://www.aeon.co.jp)? How about the Post Office (http://www.post.japanpost.jp/index.html)? The Yelp of Japan, Tabelog, looks almost exactly like Yelp.com (https://tabelog.com/ - the English version looks less modern, click on 日本語 if you get redirected).

Edit: Consider also that Yahoo.co.jp and Rakuten are an amalgamation of like 100 different services. Yahoo has news, weather, train info, maps, games, forums... If you look at the subsections, they don't have that 90s Web Portal kind of feel: https://games.yahoo.co.jp