A built up letter is a three dimensional fabricated object rather than a graphic applied to a flat panel. It has a face, a return and a back, it is assembled and welded or bonded, and it is mounted individually onto the building or a tray.
That is why they cost several times a printed panel, and why they survive decades rather than years. There is nothing to fade, peel or lift. On an office, a hotel or a restaurant they are usually the single most effective spend on the whole frontage.