I'll tell you why. It's because acting is about pretending to be someone you are not. There is no legal, moral, or any other equation that insists that people who play a part must be the same as the part they play.
In Skakespeare's time, men routinely played women. Acting has always been about people pretending to be someone else.
This is such rubbish. And, yes, I'm Jewish. People who object to not getting a part because they think that their creed or ethnicity don't understand one thing - they simply might not be that good an actor.