It has a lot of space, and, typical laptop bags have a lot of wires/metal that can confuse the issue.
There are security "friendly" bags out there that will allow you to run it through w/out removal.
But, ultimately, hidden/concealed weapons are why it's done the way it's done. Same time, if I can keep the shoes on while going through security, carry a bottle of water, full size tube of toothpaste, can't think of a reason that I shouldn't be trusted w/ the laptop
Cell Phone Guns - Netlore Archive
Deadly-Decoys