Why would China steal the personal medical information of nearly 80 million Americans? That’s been the question since the Justice Department unsealed an indictment of two Chinese nationals for the 2015 hacking attack on Anthem, the health-care giant.

Chinese cybercriminals have stolen all sorts of trade secrets from U.S. companies over the past decade or more, ranging from weapons blueprints to details for manufacturing solar panels. The thefts have been wide-ranging, but they’ve shared a common target: technical data and...