WWII was 50-90 million and WWI was 40 million. Vietnam and Korea (two conflicts that my father actually fought in) were wars, for sure, but not major wars compared to the World Wars.
There's no reasonable way to conclude these were not "major" wars in the context of US history.
The Korean War transformed the United States from a country that occupied half of Korea with the permission of a local, democratically elected government to a country that occupies half of Korea with the permission of a local, democratically elected government. Vietnam transformed the United States into a slightly less interventionist country for 20-30 years and reduced the scope of Johnson's social welfare reforms.
The World Wars also saw the wide-scale mobilization of the entire nation, including rationing, large-scale transformation of American industry to war production, coastal blackouts at night to protect shipping traffic, ubiquitous propaganda, and other measures. World War II was more disruptive to the life of the typical American at home than any other war that didn't physically take place on American soil. (In fact, parts of World War II did take place on American soil.) Korea and Vietnam--"major" wars maybe in isolation, but not at all compared to WWII.
You also forget about Korea as the start of the communist domino theory that directly led to the Vietnam intervention. And vastly understate the cultural changes that happened as a result of the Vietnam war protests.