Why?
They had their faults, but also the capabilities to win when it counted.
First of all General Macarthur.
He had a habit of getting things wrong and then blaming anyone else, it was a pattern that started well before the war.
WW1
He was divisional commander of the 42nd Division when one of the battalions suffered heavy losses he berated the commander as to why. When told that the battalion lacked artillery support he blamed the artillery commander. He however was the divisional commander and the responsibility for such a failing in planning was his.
Between the wars
At the height of the depression in 1932 thousands of ww1 Veterans with their families who were destitute set up a peaceful camp to ask for monies due to them for service in WW1. Hoover ordered him to clear the camp which he did. He used tanks, mounted cavalry and infantry with fixed bayonets resulting in the death of a number of protestors including women and children. This time he blamed the protestors saying that 90% of the protestors were communists. This was proven by the investigating authorities to be a lie. He also claimed not to have received orders telling him not to clear the camp, a clam which has not been proved. Even if this was true it does not excuse the use of tanks to break up an unarmed crowd.
WW2
When war broke out he had 8 hours warning that an attack was coming and had been expressly ordered to commence defensive and offensive operations. He did nothing. Blame this time was assigned to his air commander who insisted that he had asked permission to launch raids on the Japanese fleet but that permission had been denied. The air commander was also with some reason blamed for not dispersing his aircraft but again, Macarthur was the man in charge, gave no orders and took no action to ensure that preparations had been made.
The pre war plans called for Manila to be abandoned and not defended so no preparation had been made. He decided to fight and this resulted in a large number of US troops and civilians being trapped.
He does get credit for the island hoping campaign but that was little other option, anyone else would have the same solution.
Initially his best troops were Australian and their thanks, was to be excluded from some of the most important fighting towards the end of the war. So much so that RAAF officers threatened to resign their commissions in frustration. He was determined to make the Pacific War an American War, not an allied war.
War Crimes Trials
There were some concerns that he was personally involved in these in particular to protect the Emperor from blame. Also in some of the trials for example General Yamashita and in giving immunity to those involved in bacterial warfare tests on prisoners including allied soldiers.
Korean War.
The landing at Incheon was an undoubted success but his claim that the war was over at that point was the height of folly. He ignored a number of warning including ones from the Chinese themselves that they would get involved if the advance approached the Yalu river. He took no preparitory actions just in case the Chinese attacked, ordered no extra recce flights to cover the likely formation areas or had any back up plans, the result of which we all know.
So in brief the only bit that worked for him was the Island hopping campaign which he had no option about the rest was less than impressive.