Cards can be “print-ready” or “digital-only”. Cards that are print-ready can be printed using the in-built functionality and shipped to you. Once that card has been printed, it cannot be printed again. This means there will only ever be a limited amount of physical copies of a card in existence, preserving their rarity. Some cards will be released as digital-only and the print-ready versions are then even rarer.
To see whether your card is print-ready or digital-only, visit the card collection screen.