Well there was at least one reported incident in Japan, that battery was quietly replaced. No further info. Why not remove the dead weight of steel box. Airlines can save on fuel.
Actually there is a fair amount of information on that event - single cell failed, redesigned battery did its job of preventing the failure from cascading to adjacent cells, battery box vented fumes overboard as per design intent.
Corrective action without establishing root cause. Which troubleshooting standard is this.
I take it you're not an engineer. While it's certainly desirable to nail down root cause, the real world is not always so accommodating. Boeing had some of the worlds best experts on the subject investigating and they were unable to come up with a definitive root cause. So they did what engineers do in that case - they brainstorm everything potential cause they can come up with, and addressed those potential causes. Battery design, battery monitoring, charging/discharging, and then to cover their bases they put it in a protective steel box.
What did you expect them to do, leave the 787 fleet grounded indefinitely?
BTW, the Airbus proposal of cert by analysis is exactly the course that was followed for the original 787 cert that the FAA was so roundly criticized for. I think Lonewolf is right, the FAA is being extra cautious since the exact same thing when so badly wrong with the 787.