In addition to Hanky Panky's answer, one needs to point out that the process is not just taking the bag out. It also involves getting the appropriate gear (and personnel) back to the plane to actually be able to unload the luggage.
Also, if using containers, then you can't just get directly to any container, there's a single door through which containers are loaded/unloaded for each cargo bay, so if the bag is in the container that was loaded first, you need to unload all containers to get to it, then find the bag, and load back all the containers.
And I'm not really sure there are (always) procedures in place to know in which container the bag actually is.
Even if they get to the right container, they then need to go through all the bags in that container by checking tags one by one.
Not sure how they do things when not using containers, though. I'd hate to think they would have to unload all luggage piece by piece (checking tags along the way) until they get to the right bag...