Does swabbing work? I can imagine that the "batteries" (ie bomb) would be prepared, sealed and cleaned by one person, and then inserted into the laptop by another person so there's no contamination.
Also, the "turn on your device" thing they sometimes do offers some mitigation against this. Obviously, you can think of ways that may defeat this too, but to defeat all of the security, accurately, in one small package that doesn't look sketchy, is not easy. Fortunately, most terrorists are not the brightest people in the world.