[quitit]

>Does anyone know why this is?

Because the subtractive primaries (CMY) in reality are imperfect, they cannot absorb 100% of the light that is shone upon them. This is also hampered by whatever substrate the inks are being applied to. (Tangentially this is why there is so much fuss over Vantablack.)

>I always assumed it was just because "K" is the last letter in "black" — apparently not!

Worth noting: the term "Key" goes beyond the colour black. It's referring to the keying colour, in some scenarios this would be the darkest of whichever colours are used in printing.

Depending on the printer you're using, the K in CMYK may also not be a typical black like what you would find in a black+white office printer, but rather something that you might consider a very dark grey. Subtractive colour reproduction is a bit of a rabbit hole, you can find printers that will include a variety of colours beyond CMYK in order to help fill out gaps in the CMYK gamut. Epson have a few printers like this, some which take inks in violet, green, orange, various shades of grey, lighter versions of cyan and magenta, florescent inks etc.