What is it about Japanese ships and their curved decks?
Weight-saving measures.
Reducing the freeboard where not needed they reduced also the quantity of steel needed on specific parts of the hull,and doing so through curved sloping decks eliminated the presence of "breaks" that were widely used on older warships, which were weak spots.
More or less.
Not to mention a distinct lack of aircraft elevators (only one up forward and one aft) compared to equivalent American carriers which had three aircraft elevators, except for the Lexington-class which were converted from battlecruiser hulls and the Independence-class CVLs, which were converted from Cleveland-class CLs...
The two elevators were chosen over three because Taiho had an armoured flight deck.