Because they have been told to... and they do as they are told.

It started with the conservative talk radio bubble where rabid nutcases advocated everything just short of- and sometimes including- murder of the lefties...

Then the internet turned the volume up to 11 and allowed the nuts to create and live in an alternate universe- where the lefties are trying to turn their children gay, supporting islamic terrorism, trying to kill off christians...

There is no cure for this type of insanity... our nation may very well be doomed- there is no way forward when %35 of the population is violently delusional.