> If you're doing still photos of food, surely you can just put more light on the subject or use a longer exposure.
Longer exposure, yes, though that means having to set up a tripod for every shot which significantly increases the time it takes to get a shoot done.
Turning up the lighting is very difficult for food photography. Lights are hot and food doesn't last long under studio lights. (That's also why a lot of food photography has such shallow depth of field — a wider aperture lets in more light.)