It's not just security theater. It shifts the attack vector entirely. Instead of airplanes as weapons that could be used to kill thousands, terrorists can blow up a few hundred people.
Those checkpoints are only there to provide a soft target instead of letting it be a plane.
I agree. Sure you can still get weapons through screening, in fact I've accidentally done it twice with like 4" pocket knives, but not sure what the odds are. A lot of the "security theater" argument seems to be annoyance at having to go through TSA, cause what's the alternative, just barely screen people like before?