Casinos don't like to see cards laying around because it tends to make them seem worthless to newer players, who may decide to not bother with using them.

Many of the newer machines will gobble up the card if there is no play on it after several minutes. You can tell when a machine is ready to take one by watching the light(s) around/near the card slot:
RED: No card in slot
GREEN: Active card in slot
FLASHING RED: Inactive card about to be gobbled up.