Counterpoint, consider too that many or most petty thieves are lazy opportunists. If a thief can make off with the low-hanging fruit of an unsecured charger, then this is easy money for them. Breaking windows and rifling through a car is what many thieves do, but there's cost and risk to this move as well and Sentry-mode is something that thieves might be aware of. Break-in noise, especially on a Tesla, is going to start a few timers in the thief's head. It's higher stakes and higher risk. The thief may also understand that there's nothing in the car to steal if they took a peek in through the windows first.
To observers, someone taking off a charger with no other ruckus could be seen as legit compared to someone breaking a window or rifling through a car with busted windows and glass everywhere, not to mention the secret (or not) alarms that might trigger an angry owner with untold means of defense and/or police involvement.
Finally, if caught, stealing a charger carries less penalty (no damage to car) than a full smash and grab. This might weigh into a thief's cost/benefit calculus as well. But really, it probably comes down to what is easiest and most profitable, with the least risk.