Because most cheap cameras are stupid and will adjust themselves to suit the white background rather than the black cat. Multiple similar-colored cats are a problem but not an insurmountable one. Shoot a lot of pictures . Film is cheap. Cameras are cheap.
Processing is cheap.
If you're worried about any of these costs, consider a digital camera. But not all of your shots will be golden. Most of your shots won't be. But the more you take, the more you'll have to choose from and you won't be forced to use an awful picture on your site. Another alternative, particularly if you have a video camera, is a Snappy or other frame-grabbing type of hardware.