Modern gaming keyboards come w/ full n-key rollover over USB. My Noppoo Choc Mini is capable of full NKRO whereas my Model M can't do the same over PS/2.

USB doesn't impose a simultaneous key press limit that low, the keyboard does. The 'Report count' field of the 'Key codes' usage page of the keyboard's USB endpoint descriptor determines the maximum number of simultaneous key presses. This field is a byte, so it could be up to 255.

The keyboard endpoint example in the USB HID spec does have 'Report count' set to 6.