"Because USB keyboards don't support arbitrary numbers of keys being pressed at once."

There is actually nothing in the USB or HID specifications preventing USB keyboards from supporting n-key rollover (when using the report protocol -- keyboards using the boot protocol are limited to a 6-key rollover). The reason most don't is simply to reduce cost and complexity. A sufficiently motivated person could build a USB-compliant keyboard that supported an arbitrary number of simultaneoue keypresses, and some do (e.g. http://www.maxkeyboard.com/max-keyboard-nighthawk-x9-red-bac...).