Why do brushes have to be made of animal hair?

The writing brush was invented in ancient times, and the hair on plants can't be used, so animal hair is used as the hair of the writing brush. Commonly used hair includes wolf hair, mouse hair, wool and so on. Until today, artificial materials such as artificial fibers have been very popular, but there is a reason why animal hair is still used to make pen hair. First of all, the brush absorbs ink by capillary phenomenon. When hair is removed, it can be found that the surface of hair is rough, which further promotes capillary action. However, the surface of man-made fiber and other materials is smooth now, and it can't absorb ink at all. Using animal hair is easy to obtain and natural. There are different types of brushes, such as large letters, middle letters and small letters. The brushes used for drawing, writing, big characters and small characters are all different, and the strokes made of animal hair are more natural. So, if we want to cancel using animal hair as a brush, I think we may accept it, but calligraphers don't.