Several reasons...

(1) My ancient (1999) brick phone is indestructible, whereas most people I know who have a cam phone seem to have trouble with it. (Either the camera part or the phone part or both.)

(2) With the cam phones, you rely on the network and (in most cases) pay to send your pics through the network. I like to take my digicam, fire off dozens (or hundreds) of pics, sift through them after I've gotten home, and keep only the good ones.

(3) The midrange digital cameras have a full feature set that the phones will probably never have. (Whether the shooter knows how to use them is another story.) I can see a day where the cell phone might replace the point-and-shoot party-pic camera, but I think I will want to have a more powerful photography tool. I say "I think I will" because I haven't bought one yet and I am still trying to decide if I want to commit the effort/time needed to learn how to use a good camera.