There are many reasons for this. As you mentioned, Japan is the third largest GDP country in the world, but half of its GDP comes from personal consumption. The median salary of the Japanese has decreased by 10% over the past 30 years. In contrast, the median salary of Americans has increased 2.3 times. Furthermore, Japan is experiencing a rapid increase in social security costs due to the aging of the population. In addition, Japan has introduced MMT as a policy. The result is dilution of the yen, which may cause the value of the yen to plummet in the future. Japan is a resource-poor country. Naturally, a weaker Yen will increase the price of imported goods, but the Japanese people, with no salary increase and increased social security costs, do not have the economic capacity to do so. As a result, Japan's personal consumption will decline and GDP will also fall.
*GDP Gross Domestic Product *MMT Modern Monetary Theory