“They didn’t want to” is hiding some of the complexity behind it. Vietnam had turned into a political crisis at home; there were widespread protests and lots of people openly dodging the draft. Grunts would sometimes “frag” commanders who pushed them into overly risky missions. Running up the casualties would have created a real risk of a violent revolt at home and mutiny among the soldiers. The US as a society was likely incapable of winning the war, so it turned into a defeat.

There may have been a window earlier on when greater commitment could have won the war or frozen the conflict, via invasion of North Vietnam, but not once the public turned against it. But this came with a big risk of direct intervention from China or the Soviet Union, as happened in Korea. In Korea, Chinese intervention led to a stalemate.

> WWII proves the US is perfectly willing to suffer at least 5x the casualties of Vietnam in order to achieve their objectives.

The US today isn't the US on the dawn of WWII, and this war is unlike WWII as a whole.

Domestic political support for the war is so low due to the huge costs (of the war itself and on the American pocketbook). Furthermore, due to the open and blatant corruption in the current administration, and the passing of economic policies that increase the struggle of everyday people while enriching the billionaire class, there has been a huge loss of trust in the US federal government in general.

Contrast this with the establishment of the Social Security act and WPA before WWII, which greatly increased the public's trust/confidence in the federal government to secure their welfare in the wake of the Great Depression, and made them willing to sacrifice for their country.

Finally, unlike WWII, this war didn't start with any attack by Iran on US territory, or attacks on closely allied countries. To the extent that allies have been attacked, it has been in response to US aggression, further undermining the public's will to commit American lives to further conflict.