Why are laptop computers usually powered around 20V DC?
Two reasons. First, often the cell stack is already close to or well above 12V so internally the laptop would have to step up the 12V charge. Pack voltages of 11.1 and 14.4V are common. Charging these voltages is easier to manage with a step-down type regulator vs. a step-up or buck-boost.

As a couple of examples, a Li-ion 11.1V (3s) battery will have a topping-charge voltage of 3*4.2=12.6V. A 14.4V (4s) battery pack will be 16.8V.

Second, going in with higher voltage allows the charge cord and PSU to be more compact as the required currents are lower.