One reason is that printers inks do not have very dence pigment content. Pigment is the expensive part of paint/ink. Also, printers ink is very transparent, alm

One reason is that printers inks do not have very dence pigment content. Pigment is the expensive part of paint/ink. Also, printers ink is very transparent, almost like a staining agent. CMKY is basically a variant of the painter's RYB color space. A CMY mix using high quality painter's paints will get closer to black. However, generally mixing a black from primaries is waste of energy. Easier to mix from secondaries (e.g. an Alizarin Crimson with Ultramarine Blue).