Angle sensing IC's tend to need to be on the end of the shaft they sense, which can make some packaging and assembly headaches.

I personally am surprised they don't put an accelerometer in both halves of the laptop and use math to calculate the angle based on gravity.

They only need to be co-linear to the shaft if you care about accurate measurements, such as in a motor controller. I doubt the error introduced by being off-axis would make much difference in this application.

There are also packaging considerations when putting a hall sensor elsewhere. Packaging it in the hinge has the advantage you can use the same hinge and sensor setup in all laptop models. Packaging the sensor elsewhere means custom packaging setups for each laptop to work around all the other components in the body of the machine. Doing the extra work for packaging in the hinge once is probably quite a bit cheaper than having to constantly redo the packaging work in every new model.