They have been searching. They searched the Anthony home. They searched places Casey indicated were possibilities. In the Laci Peterson case, LE searched in the places that Scott first indicated (the park where she would walk, for example). It was receipts that led them to look in the bay. In effect, they had a way to narrow down where to search. In a city where the landscape is littered with hundreds of waterways, lakes, swamps, woods.....they can only, because they ARE limited, search where they have information a search there might produce evidence. Though we would love it to be so, they must also deal with the rest of the greater Orlando public's needs.
This is one reason why Tim Miller's group is invaluable and should be given donations. They can put "boots on the ground" so that the LE are not stretched beyond their capabilities.
Never assume that LE does not care. I have found that the men and women who have to work a missing child case care more than we will ever know or could ever understand.