You only get brown by mixing all colours together when you're talking about pigments. (Mixing all colours together in visible light produces white). If the CMY pigments you had were absolutely perfect then mixing them together would produce black, since the pigments would absorb all incident light--however, those pigments are *not* perfect, which is why you get a sort of muddy brown colour instead and why they have to add the black ink to provide better blacks in the image. (Many years ago I had a very cheap inkjet printer that did *not* have a separate black ink, and as you might expect, the results of printing anything to it were not great).