As I understand it the problem is that there would have to be some re-engineering of the 22DDH's and the two Hyuga's would need quite a bit of work - unspecified but we know what the costs of even thinking of re-engineering the RN carriers has been............
Since the Japanese Navy has based its whole strategy without their own carrier strike arm it would be a major change and it would only give them a small number of aircraft - maybe 5-6 per deck. Probably better to bulk up their destroyer force and their ability to deploy Marines in the offshore islands