German tickets purchased online through DB offer a selection of what type of ID you will bring, and passport is not on the list, so most Americans have to choose the credit card.
Quoting https://www.bahn.com/en/view/booking-information/booking/online-ticket.shtml :
The following means of identification are accepted during ticket inspection:
• German ID card
• German passport
• Children's passport
• Electronic residence permit
• Asylum registration confirmation (BüMA),
• European ID card
• International passport
• BahnCard (in accordance with the Conditions of Carriage, valid only in conjunction with a photo ID, which may have to be presented on occasion).
Note: driver's licenses are not accepted as a means of identification during ticket inspection.