the thinking is phoneposters can't provide quality content because of the difficulty of posting via phone.
you usually won't have good pics, or original pics in your case. you also generally won't be able to write posts that are longer than a sentence or two, so the quality of your posts can't be that good. also posts will be tougher to read for you. plus phoneposters presumably use phones to access the site at times when they're on the move or doing something else, so they'll be preoccupied and and only interested in making a quick shitpost, as opposed to engaging in quality discusion