Because we know how to pronounce German names properly?
I caught part of
Bill & Ted's Excellent Adventure on TV the other day and that reminded me of their mispronunciation of names like Beethoven. But does anyone mispronounce names like that except for comic effect.
As for Robert Wagner, anyone can pronounce their own name however they want. The normal rules of English pronunciation (and all the normal exceptions to those rules) don't apply to real names, people or places.
What about those modern dance troupes the Cholmondleys (pronounced "Chumleys") and the Featherstonehaughs (pronounced "Fanshaws")?
They are both based on British family names where the pronunciation doesn't bear much relation to how they are spelt
Steve