The trouble is, the composition of a particular dish can change from day to day. Often suppliers will source different ingredients depending on what's cheapest on the market at the time. Packaged foods might have a list of all ingredients they know about, but if they bought any of those components from another manufacturer, there could be things in there they don't know about. It may be literally impossible to provide a full list in many cases.
For most restaurants, the only truly honest answer they could give you is "we don't know." I think the only exception to that would be the rare ones that make everything in house from fresh food.