Institutions directly under the Immigration Bureau have career preparation when recruiting students. Why can't they get in?

The institution directly under the Immigration Bureau has a career establishment when recruiting students, but it cannot be admitted because there are not many positions that can be applied for. Most of the institutions directly under the Immigration Bureau can apply for undergraduate education, and there are also very few positions that require postgraduate education or above. The post said that undergraduates can apply for the exam, and the probability of final admission is graduate students or doctoral students. Postgraduate entrance examination has become a must.