The greatest generation - those who fought WW2, both on the battlefield and back here at home - were the last generation that had a winnowing of smart versus stupid.Up until the early 1900s, nature and a frontier allowed for that winnowing to occur naturally enough that it was barely noticed. The smart survived, the stupid died off, and the race was continually improved, and we were a nation of pioneers. That gave us the spirit they needed to pass along to their kids (who fought WW1) and their grandkids (who fought WW2).
But when the League of Nations failed and the United Nations outlawed war, that gave the greatest generation and their kids (the Baby Boomers*) and their grandkids (Generation "X" ) the chance to relax and since nature was conquered, and medical science was saving kids from accidents that would have killed them off in centuries past, there was suddenly no serious penalty for being stupid.
The teachers started seeing more stupid kids showing up in the classrooms, and had to learn to adapt until the stupids started becoming teachers themselves. This, perhaps not coincidentally, happened at about the same time as universities were starting to be taught by the baby boomers (the late 70s) and jokes about getting degrees for "underwater basket weaving" happened along to cover up the multiplicity of degree programs in some of the most inane areas imaginable, culminating in the current jokes about "a doctorate in Albanian Lesbian Poetry of the late Triassic".
It has gone from being merely acceptable to be stupid, through enforced association (peer pressure graduation), to being the eagerly sought for "norm" ("Everything I Need To Know I Learned In Kindergarten").
Stupidity cannot be cured with money, or through education, or by legislation. Stupidity is not a sin, the victim can't help being stupid. But stupidity is the only universal capital crime; the sentence is death, there is no appeal, and execution is carried out automatically and without pity. -- Robert A. Heinlein
But since there has been no natural weeding of the stupid, they have begun to outnumber us, to the point where a majority think that money comes from the government and happiness is a big screen TV.
Idiocracy wasn't supposed to be a documentary.
* - which includes every President since Clinton, and almost every politician in Washington, DC and most State capitals