Though there's a lot of things causing the stupidity to increase, the biggest factor is the fanatical blind faith in competition, and how everything has to be settled by 'winners'. "In a society based on competition, you inevitably get fewer and fewer winners and more and more losers." -Wendell Berry
The highest paid public employee in almost every state is a sportsball coach. Public education has done an effective job of feeding the fanatical ("fan" is an abbreviation) competition in general, and fanaticisms as a whole. Social media has been feeding on the habitual Us vs Them of American culture and consumer branding. A brand is not a tribe, even if a majority believe so.