I have some reliable first hand knowledge that TSA also gets tested on narcotics and chemical componants of explosives and do fairly better (it helps that they have dogs trained to sniff out narcotics) and the process entails more than just letting the plant walk through security with contraband (the one I'm aware of, the plant had a marked TSA handler with them at all times and they had to escort them at all points behind the security checkpoint for cleared luggage. Since they were training dogs, a person with a bright yellow safety vest with the letters TSA isn't going to tip off the dog that they should look there because like most members of the canine family, TSA dogs can't read. They also don't work with the handler daily, so they aren't going to look near them out of familiarity... the plant will look like a typical passenger and is usually a volunteer from another entire Government agency with a local office to the airport. (so they're reasonably reliable, but have now working experience in TSA or DHS to affect the test. The handler is more there to make sure the contraband isn't stolen from the plant, rather than any distrust of the plant themselves. The plant doesn't have a valid ticket for a flight and they already know where they live because the same boss sighs the checks for them, so it's not like they're gonna get far with illicit goods. They don't want a random passenger picking it up unknowingly... or worse, knowingly.).
Most ports of entry have several layers of checks, both passive and active and both visible and not visible so there's not a full accounting of what they use. While its assumed that the check point and lines for a pre-fight groping are "security theater", its not the only way security stops the bad actors. If anything its an Active Visible security measure. Just because it doesn't reliabley catch contraband doesn't mean it's not doing it's job... the "groping" jokes go a long way to discourage people from even trying funny stuff that is legal, or not paying close attention to the rules... or even oversharing (TSA pre-check lines? Yeah, for a small fee and surrendering information about you that the TSA wouldn't ask for in the normal process nets you a shorter line (when open) and the ability to leave your shoes on for the entire experience. You don't need a warrent if they want to give it to you and ain't no rule that says you can't make some money off the process.).