I think that's the exact scenario they tried to avoid. The lightning cable is actually pretty stiff, it won't take long for it to break if one could use it pluged in like a wired mouse.

I dislike most things Apple has done recently. But I can understand this one.

It's fine for the trackpad, designed and released together with the mouse, to have the lightning port on the back and be used plugged in, because it doesn't move so there shouldn't be much stress on the cable.