This is something I've always wondered. During the island hopping campaigns where the U.S. was on the offensive, the Japanese seem to consistently have higher casualties. Not just higher dead which would make sense given Imperial Japans glorification of dying and the encouragement of suicide or suicidal charges, but overall more casualties. As far as I know the only battle where the U.S. suffered more casualties than the Japanese is Iwo Jima. This despite the fact that they were consistently fighting a defensive battle from well prepared positions. I mean, isn't that the whole point of defensive positions, to inflict more casualties than you receive? How is it possible that the Japanese with time to prepare and especially in the latter stages of the war, with well prepared defense in depth positions that were well camouflaged and impervious to much of the pre-invasion bombardment were unable to inflict more casualties on an invader that was pretty consistently exposed and out in the open. Especially when in the later stages of the war they had abandoned Banzai charges or other impractical tactics. I'm thinking here of places like Peleliu or Okinawa. Battles where the Japanese were prepared very much like Iwo Jima. With vast cave systems hiding both troops and artillery and anywhere on the island pre-registered for their guns. From what I've read, both on Okinawa and Peleliu many marines report barely even seeing a Japanese soldier. Just looking at those figures it paints the Japanese as somehow incompetent or the Marines as really amazing fighters or some mixture of both and while I'm not disputing the combat prowess of the Marines or Army they were still human and the Japanese are generally considered a tough and competent opponent even if hobbled by supply shortages and the mastery of the sea and air enjoyed by the U.S. I mean that was part of the whole thing with the First World War was that everyone had such good defense in depth positions that the attackers almost always suffered greater casualties than the defenders and in fact were unable to make much progress until the end of the war.
Let’s first start with the definition of the word casualty.
Casualty simply means, in military terms, someone who, for various potential reasons, is rendered unfit for continued service, combat or otherwise.
These reasons include but are not limited to: death, wounds sustained in battle, injuries caused by accidents, disease, starvation, fatigue, physical disability, psychological instability, and capture by the enemy
To be clear, KIA numbers are included in casualty numbers.
The answer to your question is something you already wrote about in your question: Japanese forces were woefully undersupplied and underfed.
If nothing else, this would cause casualties (and deaths) from disease and starvation. Even with firearms, severe illness and bodily infirmity can keep soldiers out of battle.
Japanese fortifications were indeed very fearsome indeed. Many Allied planners, when looking at the scale of defensive works they would be facing were appalled at even imagining the casualties defenders could inflict on even the most organized assault.
But when manned by forces who were already stretched to their limits by limited supplies of food, medical supplies, and ammunition, they were not as great a force multiplier as they were on paper. Disease and famine greatly reduced the effectiveness of even the ablest Japanese units.
Also, I should mention that banzai charges that you mention were not ‘tactics’, at least not the kind that were put into effect in an organized, intentional way. They were heavily discouraged by command and were generally a last resort of field commanders who had reached the end of their rope.
I wrote extensively about the difference between an organized bayonet charge and the desperate charges of the Japanese forces in another post that I can link to if you are interested.
Another thing I should mention is that the idea that higher death rate makes sense because of encouragement for suicide is also a bit mislead. Japanese commanders didn’t encourage suicide per se. Rigidity of command and doctrine often forced commanders to defend or attack even at the great cost of life. For example, doctrine often compelled commanders who had their communications cut off to stay in place and defend their positions until they regained contact with command or were all dead.
You can see how easily it was for Japanese forces to fall into the trap of inflexible command structure and doctrine that lead to their seemingly suicidal tactical and strategic decisions.
But Japanese soldiers were often no less eager than their Allied counterparts to return home to their loved ones alive and well.
Even officers who were heavily indoctrinated during their time in war academies had great misgivings and could see the waste of throwing away manpower when it was so desperately needed.