The real reason is that Americans have a relatively clear and baseline concept of what constitutes being an American, and therefor we can allow for more diversity. Because we have a firm civic identity that's tied to our founding documents, we can clearly say "These are American Values", and so we can allow people to be asian in some contexts, catholic in others, etc, because we have fair certainty that that doesn't conflict. "German Values" is much more vague, and therefor harder to establish a baseline. It encompasses numerous fields and any attempt will result in a bunch of stuff being thrown in to any attempt to define "Germanness" or "Frenchness"