because it's not a real work out.
a real workout involves straining your heart and muscles so they are forced to become stronger to deal with the load. that is where the real health benefits come in. light exercise does none of that. all it really does it burn some extra calories. for light exercise to be effective it has to be endurance, as in hours and hours of it.
people may not like that fact, but it's how biology works. it's not exactly convenient.