Why was IJN apparently inactive mid-war?

Carrier groups were pretty much exhausted on both sides after Santa Cruz in October 1942. 1943 was used to regroup.

The Japanese had few carriers and fewer pilots. The USN had almost no carriers.

In the sense of what you're asking it wouldn't have been obvious, given the fog of war, to pursue offensive or diversion actions in 1943, the IJN just didn't have the oomph left in it. Now you can say "they should have done something", but equivalently many of us could now also say: "they should have surrendered".

Although the Battle of Santa Cruz was a tactical victory for the Japanese in terms of ships sunk, it came at a high cost for their naval forces, as Jun'yō was the only active aircraft carrier left to challenge Enterprise or Henderson Field for the remainder of the Guadalcanal campaign.[81] Zuikaku, despite being undamaged and having recovered the aircraft from the two damaged carriers, returned to home islands via Truk for training and aircraft ferrying duties, returning to the South Pacific only in February 1943 to cover the evacuation of Japanese ground forces from Guadalcanal.[82] Both damaged carriers were forced to return to Japan for extensive repairs and refitting. After repair, Zuihō returned to Truk in late January 1943. Shōkaku was under repair until March 1943 and did not return to the front until July 1943, when she was reunited with Zuikaku at Truk.[83]

The most significant losses for the Japanese Navy were in aircrew. The U.S. lost 81 of the 175 aircraft that were available at the start of the battle; of these, 33 were fighters, 28 were dive-bombers, and 20 were torpedo bombers. Only 26 pilots and aircrew members were lost, though.[84] The Japanese fared much worse, especially in airmen; in addition to losing 99 aircraft of the 203 involved in the battle, they lost 148 pilots and aircrew members, including two dive bomber group leaders, three torpedo squadron leaders, and eighteen other section or flight leaders.[85] The most notable casualties were the commanders of the first two strikes – Murata and Seki. Forty-nine percent of the Japanese torpedo bomber aircrews involved in the battle were killed, along with 39% of the dive bomber crews and 20% of the fighter pilots.[86] The Japanese lost more aircrew at Santa Cruz than they had lost in each of the three previous carrier battles at Coral Sea (90), Midway (110), and Eastern Solomons (61). By the end of the Santa Cruz battle, at least 409 of the 765 elite Japanese carrier aviators who had participated in the attack on Pearl Harbor were dead.[87] Having lost so many of its veteran carrier aircrew, and with no quick way to replace them—because of an institutionalized limited capacity in its naval aircrew training programs and an absence of trained reserves—the undamaged Zuikaku and Jun'yō were also forced to return to Japan because of the scarcity of trained aircrew to man their air groups. Although the Japanese carriers returned to Truk by the summer of 1943, they played no further offensive role in the Solomon Islands campaign.[78][88]

The IJN wasn't very clever with high tempo pilot training and replacements and their return in 1944 @ Philippine Sea was perhaps premature:

The aerial part of the battle was nicknamed the Great Marianas Turkey Shoot by American aviators

Short answer:

IJN was not able in 1943 to find the correct opportunity for the decisive battle. So IJN denied any big involvement of her ships. This was a fantastic strategic mistake.

Long answer:

After the loss of Guadalcanl, Japanese strategy focused on defending the Solomons againt US attacks, with land-based air power. This was a fail. Both USN and IJN at that time did not involve big ships because of lack of opporunities and heavy, very heavy losses in 1942.

Later in 1943, the IJN saw her bases in Center Pacific isolated by carrier forces attacks, that destroyed air power on its bases: IJN tried only once to react and missed the US forces. Anyway this was not the sort of decisive battles IJN looked for: Japan strategy before the war was to fight an ennemy force that had made a long path with much attrition.

Later on, IJN tried to reinforce Rabaul: but she faced opposition in the forms of air raids on Rabaul: the inability to oppose to those air raids forced her to put back her ships.

So, overall, IJN could not find:

• Opportunities according to her criteria in the Center Pacific
• Good ways to defend the Solomons