There's a couple reasons that I know of, plus I suspect it may be cheaper for the manufacturer. One is that the media type selection in the printer driver (especially glossy vs. matte) changes the ink composition used for black, and you wouldn't have the ability to adjust that if it was premixed. Another is that "automatic" use of color inks for black is an RGB thing; when printing CMYK the same thing is done but it's actually part of the original data. That is, a "deep" black in a CMYK image will have non-zero CMY. Another way to put this is that the whole "we have to use color ink to produce black" is basically an artifact of a mapping problem between how RGB additive-color and CMYK subtractive-color look for black. If you prepare a CMYK graphic you can put down a sample of "100% black" or CMYK 0,0,0,100 and whatever your editing tool considers "black," like CMYK 60,40,40,100, and you'll find that they look quite different printed. But people working in the CMYK space expect to be able to control that to their preferences; people printing documents get it done automatically as a convenience.

Photo-quality inkjet printers sometimes use two different black cartridges, I'm not sure what exactly goes into the composition of the two. Art reproduction inkjet printers (giclee) can use 10, 11, even 12 different pigments to get optimum reproduction across the whole gamut. It gets very technical.