They weren’t weak, they were minimaxed for attack.
IJN Shokaku
The Japanese carriers were designed with one task in mind only: destroy the enemy before they can hit back. They carried the largest possible airwing, with the largest supply of fuel and ammunition that could still be reasonably carried, with the best possible pilots. Everything was optimized for their offensive operations, down to some ships being explicitly designed to operate in pairs with planes taking off in opposite directions to facilitate easier operations.
IJN Kaga and IJN Akagi were mirrored, so planes from one could take off to the right and from the other to the left. This marginally improved air operations at the cost of cross-ship capability
American carriers opted for a balanced approach, they were strong offensively but were still able to absorb a punch or two and keep on fighting. They lacked an armored flight deck and could (in principle) be disabled by a single hit, but were sturdy enough not to sink and had competent damage control teams that restored enough capability for the ship to finish the battle before retiring to port for repairs. Their air wings were almost as large as the Japanese and they had somewhat more refuelling requirements.
The British took this a few steps further and made their flight decks out of steel. This sacrificed precious hangar space and made their air wings noticably smaller, but the carriers could be hit several times and keep on fighting. HMS Illustrious absorbed more hits than the entire Kido Butai did at Midway and made to port under her own power, whereas all four Japanese carriers succumbed to bombs. Later, at Formosa and Iwo Jima, the British carriers proved invaluable to the Allies, because their armored decks meant they shrugged off Kamikaze strikes and continued operations with minimal interruption whereas a single Kamikaze strike knocked the US carrier out of battle. Still, their smaller air wings meant there is no clear answer as to whether American or British approach was better for the Pacific.
For the Atlantic and especially Mediterranean there is no question, the British got it right. For the Pacific the Japanese had it wrong, in no small part due to their cultural preference for attack over defence. Both American and British philosophies worked well enough (arguably, American was better until Kamikazes, and even subsequently).
Source: Tomaž Vargazon (Quora)