Here's my view on this (btw I'm a very amateur military historian and i know most of my stuff reading books so i might make some mistakes):
Firstly waging war on the islands held by Japan causes lots of deaths. Amphibious landings are known to be costly to the invaders. So they're causing lots of deaths. Secondly waging war on islands is diminishing a major advantage the US has on Japan: armor. I know for a fact that the Japanese did not have much of an answer even to the American Sherman. On an island, armor is far less effective and thus they are losing a big tactical and strategic advantage they have.
Alternative: Attack japanese occupied territory in China. Doesn't even need an amphibious landing as the Chinese forces still occupied coastline. Once they're in China, the US army can use their stronger tanks and army and push back the Japanese and then attack their homeland using China or even Korea as a base.
Furthermore the US lost countless lives attacking Iwo Jima and Okinawa as they wanted to use them as a stepping stone to attack Japan. But once they liberate China (which i think would be easier than liberating the pacific) wouldn't China be even a better base of operations?
Why didn't the US attack Japan from China instead of the pacific? In my eyes I think this would end the war quicker and with fewer deaths on the American and Chinese part. I know that some of the islands were US territory but once the Japanese are defeated from China, wouldn't the liberation of those islands be an inevitable consequence? And this does not even account for the fact that the Chinese army was doing a mediocre job at resisting the Japanese already so the US Army would easily tip the balance. Whereas the Japanese Army was mostly uncontested in the Islands that it held.
Logistics.
Let me pose a question for you: How would you supply an attack through China?
You couldn't send convoys straight west across the Pacific -- that would put you right among the islands you were trying to avoid. You could ship supplies south to Australia, but then where would they go? Japanese forces occupied Burma in 1942 and were advancing into India, cutting the Burma Road.
There were no routes across the Himalayas except for air routes, and any land route to China would pass through Japanese territory.
Take a look at this map from July 1942, and you'll see what I mean. Your options are kind of limited.
In May 1943, the U.S. Joint Chiefs of Staff (after consulting with America's allies) finished JCS 287/1, a document that outlined America's strategic plan for the defeat of Japan.
The culminating point of that plan was the invasion of Japan, assisted by British and Chinese forces. Reaching that point would require a long campaign, the United States believed.
In 1943, the United States planned to advance across the central Pacific and invade Formosa (Taiwan), using that as a staging base for operations in China and as a bomber base against Japan. The Joint Chiefs believed that intense bombardment would be needed to weaken Japan before invasion. That would require bomber bases in China and in the Pacific, it would require a tight submarine blockade, and it would require Chinese and British support.
As events developed, the plan changed.
Gen. Douglas MacArthur, who commanded U.S. Army forces in the southwest Pacific, campaigned strongly for an invasion of the Philippines instead of Formosa. This was partly personal. He had pledged to return to the Philippines and liberate those islands from the Japanese. Practically, he believed that bases in Luzon would be needed to support an invasion of Formosa, as Japanese defenses there were thought to be tough.
Those thoughts were realized as intelligence was gathered ahead of the planned invasion. In response, Admiral Chester Nimitz (who supported the Formosa invasion) scaled back his plans, proposing instead to only seize half the island. This was a less attractive option, since it meant Allied soldiers would be vulnerable to a constant, bleeding engagement on the island. Army planners also believed that Formosa's proximity to China would allow the Japanese to continuously bring soldiers over the Taiwan Strait, despite American efforts.
The Formosa operation was much larger than the Luzon invasion, and the latter seemed like the better option, particularly after Japanese soldiers overran Chinese air bases that had been hosting the bombers of the Fourteenth Air Force. Those air bases had been earmarked for support of the Formosa invasion.
As time went on, the situation in China looked less promising, and the situation in the Philippines and elsewhere in the Pacific became more promising.
As the United States captured islands within adequate bombing range of Japan, the need for air bases in China became redundant. By 1945, there was no longer a need to send U.S. troops to China.
That's an interesting idea. There are two reasons why it wasn't used.
Logistics, or "how do you move American soldiers from America to China?" To be blunt, there is no easy way to do this without going through the Japanese-held areas of the Pacific ocean. Yes, there were operations in China through India and Burma, but to move huge amounts of men and materiel there via the Atlantic, around Africa and into the Indian Ocean would be a nightmare. Keeping them supplied the same way would be even worse.
While the US did have a large advantage in tank quality (though Japanese tanks were good for what they were designed for: use against infantry), and this was not taken advantage of as much as it could have been if attacking through China, the US had other, larger, advantages that it DID take advantage of. For example, the sheer number of aircraft and fast aircraft carriers and the attendant navy that could appear pretty much where it wanted when it wanted. It would proceed to kick in the teeth of its target, then disappear. Then would come the invasion, and then would come the land-based air. Then it would repeat all over again.
This was called island-hopping, advancing by large steps, ignoring islands that weren't threats, isolating those that could be starved out, keeping what they need to make the next big jump.
Really, it comes down to supplies. It'd be a nightmare to keep a large American/Allied army in the field in China, when it was much easier to supply the island-hopping campaign... which, it must be pointed out, WORKED WELL.
The US advantage of armor alone isn't enough reason to create a logistical nightmare, not when other advantages are easier to use.
