Guesses, Running Speculations, and Shots in the Dark

HAL (the Hardware Abstraction Layer) is the "close to the metal" device driver interface to directX. This is what puts the "direct" in directX.

GDI (Graphical Device Interface) is the slow interface most win applications use to talk with the mouse. It has only one cursor, which means it needs only one mouse: the "system mouse". This virtual mouse is the composite of all mice connected to the computer. Vendors want their drivers to do well here.

???The GDI and HAL use different drivers for the same device.???

DirectX 3 and before didn't have HAL device drivers for any input device (mouse, keyboard, or joystick) because DirectInput3 was just dressings for the GDI. Only the old style GDI device drivers were needed.

DirectX 5 -7 didn't use an HAL device driver for mice because DirectInput was still just a speeded up dressing to the GDI system mouse. (Joysticks did need a HAL driver to get the full dx features.) With still just the GDI system mouse, directX5-7 only saw one mouse. (true for dx5, but I don't know for dx6 or dx7)

???DirectX 8 still does not need a HAL driver for each mouse, but can take full advantage if the mouse has one. When the mouse does not have a HAL driver, it can only be seen through the system mouse.???

???If any input device has a HAL driver, directX 8 sees it as a separate device.???

???All USB devices already need a HAL driver to talk to the computer.???

???Manufactures of serial and Ps/2 mice wouldn't think outside the box (of only one mouse per computer), and saw no reason to write a HAL driver for their serial and Ps/2 mice. They wouldn't have written one for their USB mice either if they didn't already need to.???

Therefore, if the manufactures of the serial & Ps/2 mice would add an HAL driver for their mice, these mice would also be seen as separate mice.