For the same reason we can't close our borders and tighten our immigration laws. It's called politically-correct insanity and the liberal desire to dig your own grave, all in the name of tolerance and compassion.
"By yielding to a false form of 'civility,' we sometimes allow our critics to intimidate us. As I have said, active citizens are often subject to truly vile attacks. They are branded as mean-spirited, racist, Uncle Tom, homophobic, sexist, etc. To this we often respond (if not succumb), so as not to be constantly fighting, by trying to be tolerant and nonjudgmental -- i.e. we censor ourselves. This is not civility. It is cowardice, or well-intentioned self-deception at best." -- Clarence Thomas, U.S. Supreme Court Justice