Why did the Japanese never adapt with anti-submarine tactics in response to U.S. submarine warfare in World War 2 that devastated Japanese merchant fleets?

Part of the reason was an obsession on the part of the Japanese Naval General Staff on the idea of a decisive battle - in 1890, an American strategist called Alfred Thayer Mahan published an extremely influential book called The Influence of Sea Power Upon History. This espoused the idea a single, decisive naval battle could decide the outcome of the entire war, and was widely studied in Japan.

Much of the IJN's planning before and during the war was therefore based on the idea of bringing the American fleet to such a battle somewhere in the Western pacific, with the big gun battleships of the IJN hopefully being able to triumph. In this type of short decisive conflict there is much less need for anti-submarine warfare (ASW).

There was also a cultural reason why few resources were devoted to ASW. The code of Bushido - the Samurai warrior culture which permeated the Japanese military meant that few officers found any "honor" in the relatively unglamorous work of escorting merchant ships, and placed more importance in offensive action rather than defense. Instead, Japan's destroyer's trained extensively to fight surface actions in line with the Mahanian doctrine.

Source: The Conquering Tide: War in the Pacific Islands, 1942-1944, Ian W Toll. (2015).