GGB Because their revenue is contracted and their costs are engineered to a known volume. A caterer feeding a committed headcount under a term contract knows next month's covers within a narrow band, so purchasing, labour rosters and production plans are built against certainty. A restaurant guesses demand daily and pays for the guessing. The caterer's margin per meal is thinner — but it is earned thousands of times a day with far less variance.
From The Success Behind Industrial Catering — the full reading, with the method behind this answer.