This post misses the forest for the trees. The reason for the low margins in the restaurant business is the lack of differentiation. Their products are largely commoditized, so they have no competitive advantage or market power. Peter Thiel discussed this in a lecture for YC, where he said that you can work very hard to make the best restaurant in a city, but it doesn't give you much pricing power, because the second best is still pretty good.[1]
[1] https://www.youtube.com/watch?v=3Fx5Q8xGU8k