Likely didn't want to get caught. Apparently the payoff for true narcissists is something called "narcissistic supply" and that can mean a lot of different things I guess. Some are attention, theft, admiration from others, feeling good they have everyone duped is a special "high", feeling like they are smarter or superior in other ways to everyone else, infantilizing others, feeling like the role of others is to admire them, serve them, prop up the nonexistent sense of self these people have, trying to accumulate "power", trying to be a "star" or to associate with "stars", trying to make sure people don't speak out, there are apparently a lot of reasons.
The big thing is they aren't fixable, they can't develop a conscience and according to the page they actually have differences in their brain scans than non-narcissists do. I guess the scans show a lack of brain development in the area which regulates conscience, empathy for others and other things. Apparently they can mimic it and can mimic humble too as a manipulation to keep people involved with them but never develop it. "They are so awesome", is frequently heard by people they show their "false self" mask to, they need an audience so they have to cultivate that. To fake something and steal to boot would be quite thrilling for them as it was working, a real "high" and reinforcement to them of their sense of entitlement. Some of the more ill narcissists would do something like this and think it was ok because "people would want to glorify them."