In some ways, the Surface Duo 2 Pen Cover is a bit odd. When you buy the Slim Pen 2, there is no way to recharge it unless you buy the $35 charger. That's a change from the original Slim Pen (which is still available), but there is a hidden cost: Slim Pen with charger is $150, while Slim Pen 2 without the charger is $130. Slim Pen 2 adds haptics, has a new design, and improved accuracy making the slight increase in cost worth it for some. While haptics don't work on Surface Duo 2, anecdotal reporting in forums from users says Slim Pen 2 works better than Slim Pen 1 for inking on the device.
All this means if you have Surface Duo 2 and Surface Slim Pen 2, either the Pen Cover ($65) or charger ($35) are required. At least Microsoft gives you a choice.
As to why Microsoft couldn't build wireless charging for the Slim Pen 2 into Surface Duo 2, it's a question of space. Surface Duo 2 already went from 4.8mm to 5.5mm, making it thicker and heavier than the original Surface Duo. Adding in a wireless coil would have made that even worse, as the Pen Cover demonstrates. There's also cost, as it would have driven up the price even higher, even as Surface Duo 2 is already more expensive.
Source : https://www.windowscentral.com/surface-duo-2-pen-cover-review225