That's a good question! It is possibly because the Japanese publishing industry is very conservative. In academic circles (primarily science and technology fields) there is more open sharing. See in particular CiNii (「サイニィ」・国立情報学研究所論文情報ナビゲーター）, although it is still nowhere near as open as the Western publishing industry.
It is rare to find full text articles in Japanese. The best way to obtain articles is to search the following indexes. Some might have links to the full-text, but if not, then request the articles via Inter-Library Loan. Please type in Romanazation for the Japanese language input details.