Well for one, when would the airlines have time to perform the required maintenance on them? Thats why a lot of us aircraft mechanics work the night shift. Repairing all the things the pilots and flight attendants find wrong during the course of the day, as well as basic maintenance checks.
And if the was stranded someplace else by another operator, it would be a nasty situation. You wouldn't believe how management and ops people scream when a bird is broke somewhere. Can you imagine if it were being operated by someone else, and the possibility of them not having the tooling or parts locally to make the required repairs to make it airworthy again?