Pre-training is based on a proxy for desired output not actually desired output. It’s not in the form of responses to a prompt, and 1:1 reproducing copyrighted works in production would be bad.

It’s the difference between a painter copying some work and a painter making an original piece and then get feedback on it. We consider the second trial and error because the full process is being tested not just technique.

a chef doesn't get feedback on his meal after picking up the spoon. he gets feedback when he or somebody else tastes the meal part way through and at the end.

There is more than one correct answer in reality, LLM pre-training just trains it to respond the same way as the text did.

Imagine if school only gave correct if you used exactly the same words as the book, that is not "trial and error".