Because politicians and the media figured out that the easiest way to get people to watch/vote was to appeal to their most basic hatreds and fears, and to even create new ones. Essentially imagine human manipulation as a game, and the game has been solved already. Now this can be countered by teaching people how to actually think for themselves and form their own opinions, but we constantly try and reduce access to education in the world. Were kind of just fucked until the aftermath of WW3 imo.