That's part of it. The other part is depreciation. Lenses stay in production for far longer than cameras, so there's a longer period over which to earn the money back. For example the Canon 70-200mm f/2.8 Mk II was introduced all the way back in 2010, but it's still the state of the art in telephoto zooms and therefore it is still in demand. But the cameras which were "current" at the time (5D Mk II, 7D, 50D, 550D) are all now seen as pretty dated.