A sign is a piece of design before it is a piece of aluminium. Letter spacing at three metres behaves nothing like it does on a business card, a logo that reads beautifully on screen can fall apart at distance, and a colour that looks right on a monitor can be unattainable in powder coat.
Because the studio that designs the artwork is the one that fabricates and installs it, those decisions get made once, by people who know what the material will do. There is no handing a file to a sign shop and hoping.