There is a pesky problem with teetotaling and dessert- and coffee-refusing vegans and vegetarians. There are, though, “modified” vegetarians who refuse red meat but willingly accept fish and chicken. The more dedicated among them tend to challenge the server to reveal the previous living conditions of the chicken. (There are some folk who understand that having a free-range chicken is as dangerous as having a free-range boyfriend, because you never know where he has been!) Furthermore, they may insist that the food contain no salt, no butter and, obviously, no ingredient to which they are allergic.
Preparing several vegetables and arranging them beautifully on the plate takes far more time on the part of the kitchen staff than tossing a steak on the grill and retrieving a baked potato from the oven. And despite a considerable amount of fetching and carrying, at the end of what may be a relatively small meal, the server can expect a tip, albeit calculated with excruciating accuracy.