Although this seems to have been discussed in the Diet as premise, it is not possible to acquire a doctoral degree only by acquiring the number of credits required to graduate from the doctoral course (or doctor's course). In addition to the credits, there is a degree review process that includes preparation of a doctoral thesis and a public hearing, and it is necessary to pass all thesis examinations. Therefore, if you leave school after only acquiring credits, they will only be registered as ABD (All But Dissertation) in your resume, and you will not be able to use the "Doctor" title on business cards.
First, the preparation of a doctoral thesis is different from that of a business document, and requires that empirical research be conducted based on objective facts and accumulation of experiments in addition to the thesis research, presenting theories that can explain new facts that were not reported by other researches, as well as further generalization. In addition, the important aspects that lead to the conclusion of the doctoral thesis need to be at least published for domestic and international peer-review papers (PRJ), and this does not mean that the thesis will be approved when the advisor gives his endorsement. It is necessary for the thesis to undergo a professional quality check.
Unlike the natural sciences, the threshold for the elaboration of doctoral theses in the social sciences and humanities sciences (which are areas with a high and difficult level of experiments and demonstrations) is even greater, and the completion of a doctoral thesis over three years will become difficult. Therefore, until recently, it was common for people to quit school temporarily and complete their doctoral theses within a few years, and then obtain a graduation certificate (obtain a Ph.D.). It may seem somewhat strange that it is possible to submit a thesis and graduate after quitting school, but in extreme cases it has been seen that students quit school after getting credits, and then continue to work for many years in college, being awarded the title of Ph.D. upon retirement.
According to this system, the acquisition rate of Ph.D. degrees will be low when compared to international standards, and moves have therefore been made to actively award Ph.D. degrees. However, there is an overlap of traditional circumstances where instructors who teach doctoral courses do not have a Ph.D., and only one among many students is granted a Ph.D. in a lifetime. Even now, 45% of graduate students in social sciences have opted to "quit college after getting credits" (Ministry of Education, Culture, Sports, Science and Technology fiscal year 2015 survey).