The robot would need to operate on ingredients that are within a narrow band of parameters. A cook will adjust the receipe if for example some vegetables or fruits are particularly ripe or tough or whatever, while the machine would have an incredibly hard time doing that. Realistically, the ingredients would need to be standardized (within a narrow band), which would increase the cost of final meal. Of course, fast food is practically doing that already, but in result they can only do simple meals with mediocre results.
>>burnto+(OP)
I would guess that this is why cheap packaged foods tend to be ultraprocessed. The machines need consistency to operate reliably, and the easiest way to make food consistent is to turn it into a liquid, powder, or mush.