Because there isn't a complete path between battery A and B for current to flow.Precisely the answer I was trying to prevent with my original wording
So is it a hardware limitation or something else? I'm really hoping it's not simply "that's just the way it is"
Electrostatic charge is a real force in electronics. A good example is the gate terminal of a insulated gate MOSFET transistor. Because it's insulated from the other terminals there is no steady DC current flow needed to turn on the device. However to charge that gate terminal still requires a 'complete circuit' in that the applied positive gate voltage must have it's negative terminal wired to the source terminal. This creates the 'complete circuit' so that the positive voltage potential is felt between the gate and source of the mosfet and a charge voltage can then be developed.
It's the same thing as why a bird can land on an energized 20,000volt power wire and not feel a thing, because there is no complete current path back to the power wires return potential. Connecting just the + of one battery to the - on another battery just does not create a potential difference between them. The + terminal on a battery means absolutely nothing without referencing the same battery's - terminal.