Most of the scanning rules rules are motivated by psychological effects ('whatever makes the public feel more secure'), not for their efficiency - in the US alone, in average 2-3 hand guns are known to be missed per day in the scanning process.
TSA Pre, CLEAR, etc., also allow scanning with the laptops (and anything else) still inside the bags.
If you peek at the monitors, you can see the interior life of aluminium laptops (and suitcases) quite well - and it is rather complicated. So if you would carry a computer (and maybe some cables, a mouse, an adapter, a camera, and some spare batteries) and a hand gun or a bomb, it is hard to see what belongs to and is inside the computer, and what is outside and worthy looking at. Therefore, they ask to take separate various of those devices.