Quite simple. The odds your car and tag are being seen in public by an interested thief are pretty damn slim. Car thieves who are interested in our cars aren't wasting time driving around the streets or cruising parking lots hoping to stumble onto what they are looking for. They use the internet. They Google "red 1969 Dodge Charger", select Images, and start shopping. If you have such a car, and have pics of it with the tag showing, they can find out right where to go to get it.
And how do they find out? Because as we've been learning for years, we're not paying the folks at the DMV enough money for them to pass up a quick payday. If you have a "client" who's looking for a high-end car and willing to pay you say $50,000 for it, then offering some clerk at DMV $2,500 for some info is a no brainer. There have been quite a few DMV workers who've been arrested for providing driver info to burglars, and they still don't get the idea that burglars eventually get caught. But your better car thieves don't, so they are much less risky to do business with.