To me it's about how they're marketed. Player cards for games aren't marketed as collectibles and aren't sold as an entity unto themselves. They don't really have any inherent value and only exist so the owner can play a game. It's sort of like the difference between real money and Monopoly money.
What's amusing is that a game called Dynasty League Baseball (the best baseball sim on the planet IMHO) has player cards that are exactly the same size as baseball cards. They do that for two reasons. First, it's so baseball card storage boxes can be used to store the player cards. Second, it's so geeks like myself can put the player cards into a penny sleeve and then put a baseball card in the sleeve with it. That way when you're playing the game you can look at the baseball card to see what the player looks like.