Cause they can’t!1!

But for real, for those who are curious: the border between Germany and Poland is effectively the border between western and eastern Europe. So to Slav people Germans lived right over there, and yet spoke something incomprehensible; so we called them “mute” (in Poland at least). If I can’t understand you you are mute to me, basically. And the word for “Germans” is the same as for “Germany”, so we call the country itself mutes 😅