Not just that, but also - it's not as though there's a single Optical Mouse Interface which every mouse uses internally. There's probably hundreds of slightly d

Not just that, but also - it's not as though there's a single Optical Mouse Interface which every mouse uses internally. There's probably hundreds of slightly different sensors, all of which work differently - having a single, standard interface to them (USB HID) is a lot better than having to support every single one with its own driver. (And that's even without getting into the bandwidth issue - the camera on a modern optical mouse captures far more data than it can transfer over USB.)