Why can't I practice writing with a ballpoint pen?

In fact, there is also ballpoint pen calligraphy, which belongs to hard pen calligraphy. But the font written by ballpoint pen is different from the traditional calligraphy style. For example, pen calligraphy is also a hard pen, but because the pen tip can be opened and closed, it is still possible to write changes like brush calligraphy, but the handwriting written by ballpoint pen rarely has these changes, so it is the most effective to practice with a pen in general hard pen calligraphy.