Spelling, punctuation and grammar are at the top of my list of reasons. It seems to me that writers ought to have basic writing skills in the same way that carpenters ought to have basic carpentry skills and plumbers should have basic plumbing skills.
I have nothing against amateur writers, carpenters or plumbers. I just think it's dishonest for amateurs in any field to "hang out their shingle" for the public, so to speak, if they don't have the basic skills.
Just my two cents' worth.