That's because there are so many variables that can affect the process. These include temperature, developing time, water quality, the age of the chemistry and even the quality of the electrical supply. With digital imaging, photographers get consistent results because many of these variables are eliminated. And unlike the old darkroom process, the photographer can exercise absolute control over select areas of their images. Digital imaging software allows us to edit and change colors, sharpness, and contrast at the level of individual pixels.
Time is also an enormous factor.
Ten years ago, it meant hours of mixing chemicals, prepping the darkroom and burning and dodging in the darkroom. If you dodged and burned, the possibility of getting the same results from print to print were at best unreliable. Today, color printing still means time in front of the computer screen. But now the work is reproducible and consistent even months or years later. This is all due to advances in printing technology, allowing a wide variety of inks, papers, surfaces, and textures.