It's like taking the engine out a each car, putting it to a test bed and running it and then making a decision whether the car is good or bad based on the graphs the test bed provided.
You might have the best engine in the world, but if you put it in a shit car, the result is still bad. The seats are squeaky plastic, the infotainment is touch-only and you can't put on your seatbelt without knocking down whatever is in the cupholder.
It is a fundamentally hard problem to solve
When I use a calculator, I know exactly what it does and what it is supposed to do. It always gives me a verifiable, predictable result. If I input “8+8” 10,000x it will give me “16” 10,000x outside of incredibly fringe edge cases/bugs. I can’t say the same for LLMs
Take AI out of the picture for a moment. What makes someone a good coder? What makes someone intelligent? How do you evaluate those skills?
Of course we have standardized tests, and they're useful, but they're also imperfect. And they become especially imperfect when people start training for the tests specifically—which is, essentially, benchmaxxing.
We have never been able to quantitatively measure most skills to a high degree of accuracy, despite centuries of trying. That's not going to change now.
(I don't mean to anthropomorphize the LLMs, but I do think they're like humans in this way.)