The same reason that people insult others over the fact they adopt a different religion or that they they come from a different country or they have a different political belief or have a physical abnormality or a learning disability or use rhetoric that others disagree with or are just someone that others don't like.
As long as we are human beings and have fluidity in intelligence levels, maturity, tolerance and understanding then all the above and more will keep happening until the end of time. Countries will keep firing missiles at other countries. Politicians will keep saying one thing and doing the other. Religious dogma will continue to blight the globe and people will keep being rude and insulting to each other, in a society that is becoming less tolerant day by day.
It is a brave crusade to try and change human nature. The best we can strive for is for each and every one of us as parents to do our utmost to instil into our children a sense of worth, tolerance and understanding. To help our children to grow up with the ability to question and reason and to treat others with respect and dignity.
What a depressing answer. If humans had never changed we'd still be living in a cave, eating what we'd picked off trees or clubbed over the head and dying at the age of about 30.