1. Japan's information-dense design pre-dates the Internet. If you look at commercial fliers from the 80s (and probably before) until today, they have similar information-dense designs.
2. This design is not a result of a "critical shortage of competent IT professionals." Just because something is different doesn't mean it is wrong or the makers are incompetent. Some of these e-commerce firms have thousands of good developers. Designs are extensively A/B tested. They are common because they work.
3. Hacker News is also very info-dense and has changed little over past decade. It's not due to incompetence, but because we users like it this way. Now, imagine we had a steady steam of designers posting about how backwards we are and how they needed to "fix" the site to improve our experience. That's what it often feels like reading these articles from Japan.