iiiiiiitttttttttttt

That is NOT the reason. FFS. If that was the case, you couldn't switch to B&W only printing when you're out of one of the cartridges, and, shocker!, you almost always can (assuming that you don't have the absolutely worst printer driver in existence).

I work in commercial printing, and I print in CMYK every single day. Almost nothing is absolutely pure cyan, magenta, yellow, or black. Printing pure black ends up looking like a very washed out charcoal grey. If you want 'rich black'--which is what most people think of as black, you need to us C,M, and Y. If you had a spectrophotometer and were creating color profiles for your printer, you'd be able to very, very quickly see that. (You'd also be able to see that the colors used in most inks and toners isn't strictly linear, and that you can start getting weird 'hooking' in colors once you exceed a certain ink volume. Some inks are much worse than others in that respect.) Depending on the RIP software that you're using, and how you create the color profile for the printer, you can specify exactly where greys switch from being monochromatic (K only) to using the full gamut.

It used to be really apparent with our old Roland printers, where you could easily see the individual pixels with a magnifying glass. Now we're using printers that are higher resolution--I think 600ppi natively, but I see enough dot gain in what we're printing on that anything past 150ppi is irrelevant--you can't see them.

There's a collection of images that I have to print regularly from one of our corporate clients. This collection of images is always sent as greyscale .tif files. When you look at them on-screen, they look fine. When you print them, they're washed out. The issue is that the RIP software sees the images in greyscale, and defaults to using K only. If I convert the images to RGB (which, yes, I know, it's weird that I print in RGB when the printer is CMYK, but trust me, it improves color slightly), then the printed image looks like the image on screen.