The materials of the sofer's craft are themselves governed by Jewish law: the parchment, klaf, must come from the hide of a kosher animal prepared according to specific requirements, the quill is traditionally cut from a feather, and the ink follows a specific traditional recipe, so that the physical object a sofer produces is regulated by religious law at every stage of its manufacture, not only in the letters written on it.
Checking a completed Torah scroll for errors by eye alone is slow and imperfect even for a trained scribe, and computer-assisted proofreading, in which a scanned or transcribed copy of the text is compared letter by letter against a verified master text, has become a standard supplementary check used by many scribes and communities in recent decades, catching errors a human reader might miss without in any way replacing the requirement that the scroll itself be written entirely by hand by a qualified sofer.
The profession remains a genuine scarcity trade: training takes years, a kosher Torah scroll represents many months of a sofer's working time, and the cost of a new scroll, which can run to tens of thousands of dollars, reflects that scarcity directly, making the repair of an existing scroll, when a single letter has faded or cracked, a far more common piece of a working sofer's business than writing a new one from scratch.