Yes, that's a crucial point, and I would note, once again, that the Naylor report raised it.
The government invests in students' education for a number of years. However, scholarships don't run for the same term as their studies, as a result of which they lose their main source of funding as they approach the end of their education and have to stop focusing on their doctorate and research. You should note, incidentally, that doctoral students are involved in writing scientific articles and have been writing them for a number of years. When their term of study is longer, it costs students more to earn their degree. They may not even complete it.