star trek - How effective is the Kobayashi Maru as a training exercise when everyone knows it is an unbeatable scenario?

James T. Kirk: Bones, doesn't it bother you that no one's ever passed
the test?

Leonard 'Bones' McCoy: Jim, it's the Kobayashi Maru. NO one passes the
test, and no one goes back for seconds, let alone thirds.

There's a difference between knowing that no one has passed, and knowing that a test is impassable.

Bones' recognizes that the lack of people passing shows that failure is normal and to be accepted. While attempting is required, he does not question failure because of the ample evidence that others failed to defeat the test. Their failures demonstrate the apparent difficulty of the test to him, which ironically reinforces its validity to him.

What Kirk did was recognize that the pattern of failure implied that the test was impassable - literally designed such that passing was not possible while the test functioned properly. Once the potential for success via a fair attempt was invalidated, it allowed him to reinterpret the rules of the game - the only way to win was to cheat the cheater.

So the answer to your question is, the Kobayashi Maru still functions because people treat a test that they'll fail because it's really really hard differently than they treat an unfairly unbeatable test.