First of all, doctoral programs in mathematics-related majors are often boring and require a strong mathematical talent. This requires students to devote a lot of time and energy to research and maintain perseverance and abundant physical strength and energy under various pressures.
Secondly, it is very difficult to graduate from a doctor's degree, which may be even more difficult for the department of mathematics. In addition to publishing many SCI papers, you need to write a graduation thesis and pass the defense. This requires students to have a high degree of professionalism and rigorous research attitude.
In addition, most doctoral students in mathematics will delay graduation, and they will not be able to finish all their papers until the fourth or fifth year. This also increases the difficulty and time cost of graduating from doctoral degree in mathematics.