A laptop full of Semtex looks very different on an x-ray from a laptop full of logic board, hard drive, and dvd drive.   They're also pretty full--there's not much room to squeeze explosives into a laptop without removing those bits.
The key is the battery, which appears as a big black uniform rectangle on the machine.

In reality, X-ray machines aren't terribly good at bombs - they're designed to get the outline of metal handguns and knives - specifically, the outline of the FAA/TSA test handguns + knives.  We're wide open to any kind of attack from an inventive terrorist group with access to explosives and a few lives to spare.  Making people turn on their computer + such is one shallow measure to alleviate that.

Lucky I am not a bad guy, but as a writer of Mysteries and other fiction.  Give a mind a few minutes and you can get about 8 ounces of semtex or other nasties into a laptop and on a plane, and I am sure it is still scary to those that know how to do it for the good guys to know the bad guys can do about the same thing.

Give them time, and the only way to fly will be naked and baggageless.