Keep you busy at CBP, keep the bag from the belt when you first land... plenty of opportunities to physically attack the device when the person can't keep it on them and keep an eye on it.
And, still, at bulk these things aren't feasible, or even necessary. You're not taking your cynicism far enough. Why does the government need to sorta kinda hope that you might maybe take a flight with your gear someday when they're already capturing all your email and web browsing anyhow? It's a ridiculous theory that you're only entertaining because our trust has decayed to the point that the mere fact that something is an accusation against our government is all but proof that it must be true, no matter how silly it is.
No, the government is not going to set out to physically attack every laptop passing through an airport, because what would that give them that they don't already have, except huge operational costs and risk of detection?