Because you don't have a real picture of the German language. Honestly, I watch everything in English and I am a native German. It's simply too funny if somebody starts to speak German in a American film. Like in the series Grimm, in which they use a lot of German. It sounds ridicously funny. "Alles hat ein Ende nur die Wurst hat zwei." (Everything has an end, only a sausage has two). Good old saying that we are all annoyed of because we don't think it is funny and our grandparent have already overused it in their dementia. But it's in there in a pronunciation I didn't even correctly understand. I had an idea what he just said, but I had to wait for the English translation to be sure.
The picture people have of Germany, with its hard sounds, are often derived from Hitler's speeches and theatre. Hitler had a special kind of voice, something that came to him naturally, but others actually train for. It is that kind of voice which you use in a theatre. That's because you have to carry your voice to the last rows of seats without using a microphone. But colloquial German is losing, like every colloquial language, a lot of its hard sounds. You just skip over them, like you do in other languages. Half pronounced is enough, mostly goes ^^
So in the end, real German doesn't sound half as hard as you see in all the films. They just take that kind of German to satisfy a stereotype. Without that, people wouldn't even believe that's German.