Screwing things into the wall takes some patience, knowledge, a drill and some skill. Not everyone is comfortable with that, or has time to research different types of wall, pick the right fasteners, deal with any surprises during drilling etc.

(e.g. the walls in my (rented) apartment are mostly really solid. Some parts won't even yield to my hammer-drill. Putting up my projector was easy (hollow wall) but I spent 2+ hours putting up my projector screen on the opposite wall (even though only 2 screws).