If you look at the list of exclusions from coverage for thefts from luggage, you will see why it's not a particularly useful thing to steal people's dirty laundry. Some stations such as LGA still routinely check tags and others do it on a roving basis.

But, it is expensive and it delays passengers and passengers hate delays.

If you look at the list of exclusions from coverage, you will see that carriers, certainly on domestic services, don't cover a long list of valuables. So, stealing dirty laundry is a bit foolish. I'm not sure what the point of expensive luggage is. For international, for the most part, that is coming off a belt before Customs, so harder for a thief to reach (although pre-cleared flights do arrive into domestic terminals).