Two students write an essay. One student writes one page and the other student writes two pages. THe two students papers have, for arguments sake, the same content of ideas. The student with the one page paper is the better writer and communicator. Assigning an arbitrary length of two pages would hurt the student with the ability to write a one page paper. The student would write a good paper, and then fill it with a page of filler.
The problem is most English teachers in America have never had real world experience in written communication. They are great with explaining Frost and Shakespeare, but a business memo has to be written with length and clear communication in mind. A memo should not be a literary work of perfection. It is an entirely different skill to write a memo as opposed to writing a literary essay in secondary school or university.