Why Don’t Teachers Allow Students to Write Exams in Pencil?

I have questioned this rule time afther time, each and every time when my Business teacher hammers on the students who halfway through writing the exam get caught writing in pencil. Instead of Blue or Black ink. Then I stare at the teacher who is Furiously hitting the table, screaming, yelling an un understandable speech at an unbearable high pitch. Her swollen veins clearly visible in her Neck. What’s the point? If the student’s handwriting is clearly visible in pencil. The only reason I can think of, is they want to let us be ashamed of our thinking process. THEY WANT TO SEE OUR MISTAKES! Which I understand because it shows our thinking process, however this is not a reason to lower our self esteem.