Random checks generally work as a deterrent. Like randomly checking athletes for steroids. The risk of getting caught will deter them from juicing. But a suicidal terrorist isn't going to care much about getting caught. Sure, if you check 10% of the passengers that will stop nearly 10% of terrorist weapon smuggling, but I don't think anyone will find that an impressive accomplishment. You could check say, 30% or 50%, but then the process will be so time-consuming, you might as well just check everyone.