Because they live in Germany and that is the language they speak in Germany. What other answer do you expect?
You say (this is pronounceation): Sprech*en see dooitch: Sprechen Sie Deutsch (auctuall)*do a wierd sound like a cat hissing on chhttp://translate.google.com/#en|de|
Do you speak Spanish in Spain and Swedish in Sweden?
Sprechen Sie Deutsch? (formal) or Sprichst du Deutsch? (informal)
Who speaks German? is Wer spricht Deutsch? in German.mommy