Update thanks to thethatfox:

Physical game cards may also not actually contain the game:

Nintendo Switch 2 Game-Key Card Overview

Game-key cards are different from regular game cards, because they don’t contain the full game data. Instead, the game-key card is your “key” to downloading the full game to your system via the internet.

Update 2: There is probably a difference in Game-key cards and card that contain real game data. So we don’t know right now how often these game-key cards are used or if nintendo is using them.