I believe because they just don't need it for what they do. If a pro photographer was shooting wildlife at dawn with a flash (on the camera or their own attached flash), they would scare off the subject if their flash went off. It's also the same for sports, etc. If big flashes went off, the subject could be blinded by it. If they simply use the flash on their camera, it would not give enough light to capture further than what the flash can capture which is usually about 10 feet for an SLR. For a pro flash, the max brightness the flash would capture is 32.8 feet at it's center and about 16 feet at its peripheral (this info is from a camera spec sheet for Canon but it's not going to be much different for a Nikon or any other brands).
If they are taking an object that's non-human or an animal, the flash would bounce or reflect in it such as a car, bike or building. It would take away from their art.
Flash is good for certain types of photography like when capturing rim light and places that are dark with no reflection, and other types of art that use it.