It's more common than you think.
People who don't live in the area (Florida in particular is a vacation destination) won't bother with cards.
Also many people are superstitious. They see a correlation with losing when having their players cards in. Surprisingly these same people may let you put your card in if they feel it's good luck for them. Guaranteed if Tasha sees that $2.50 wins $2000 guy again he will beg her to put her card in while she plays.
Taxes are another reason. Some people want to hide their play from IRS or even child support.
Finally just plain laziness. People who show up and left their card at home and Don feel like wasting time at the rewards desk.