Question

Why does facial hair grow so fast???

Answer 1

In boys it's because of testosterone it could also be genes and in girls probably androgens

Answer 2

This might be a perception thing and that the facial hair is growing at the same rate as the hair on your head. Since you're constantly grooming your facial hair, even 1mm growth will affect the appearance whereas a 1mm growth on the hair on your head makes no difference because your hair is already long.