Because we've bulldozed wetlands and prairies and invested hundreds of millions of dollars in non-renewable building materials to put them there? "Save" might not be the best word here, though.
The Law of Sunk Cost: if you abandon a project, then all the money you sank into it to begin with is lost! Therefore, never abandon a project, always throw good money after bad.
Though I think we shouldn't have them in the first place.