Is it because it might be construed as tacky, and low-brow (see Denny's, Chli's, etc.)?
Is it because it would be too expensive, esp. if you change your dishes regularly?
Is it because it is considered unnecessary?
Sometimes I wonder if pictures on a menu would defuse alot of diner issues before they even start.
You would arguably know how much (or little) food your app or entree was, thereby avoiding rants of "I paid $X for this beef carpaccio and all I got was two damn small slices of beef!"
You would know the preparation of the dish, e.g. whether that burger you ordered was a "knife-and-fork" job, or something easily suited for hand-and-mouth delivery.
Curious as to your thoughts.