It takes a lot of time to do these tests, and the risk of being chosen to be tested is usually enough deterrence to prevent teams from breaking the rules. They definitely should increase the number of cars they check for plank wear now though.
They also don't check the fuel of every car, only half of the cars.
I know some of the tests are hard to perform, and not practical to do on every car. But I can't imagine how some tests, especially plank checks, aren't feasible to perform on every car.