Because you can't implant enough to be certain of causing destruction of the plane? It's an interesting question. The fluids thing is actually based on a real threat from binary liquid explosives so the ban on larger bottles is rational. The other thing is that the screening process was developed in Israel and has been found effective in identifying terrorists. It's hard to find an actual terrorist bent on killing in order to test the method but Israeli experience is a good indicator.
The point is that perhaps the way they're measuring "effectiveness" of the technique is fatally flawed, not to make a pun.