why are laptop chargers 19 volt when batteries are only half that?

One possibility is the reduce the current, and therefore the thickness (and so cost) of the connecting wires and plugs and sockets. The voltage can be reduced, and the current correspondingly increased, with relatively little loss, on the motherboard. This would be done close to the battery connector so as to reduce the trace thickness needed on the board.

Sylvia.

Yep. In the case if the Thinkpads, they both increased the voltage from 16V to 20V and changed to a thicker cable and connector. It's likely they did that because the maximum power with the new power supplies is more than double the 72W maximum used with the 16V design.