Because compilers and LLMs do different things, and what is done matters, so you can't reason by stepping from one to the other.
Compilers don't axiomatically yield derivative works, they simply in practice do because for non-trivial programs they preserve copyrightable elements of the work in the output.
So, if we will compile or decompile code using LLM instead of a compiler, then we can use the result for free?
(LLM can translate code to/from other code or to/from a machine code).
Well compilers are a mechanical transformation and if that were sufficient to free you of IP law then IP law wouldn't work.
An LLM is also a computer program which takes input and produces output related in some way to that input. However I don't think most people would view it as a "mere" mechanical transformation. One could tautologically argue that an LLM blends the user input with the training inputs which is a sort of transformation and further that the LLM itself is a computer program thus it is mechanical in nature. However it should be immediately obvious that such an overly literal interpretation is in danger of subsuming human work as well. Where the boundary lies is an unanswered question.
Related, compilers can pose a problem depending on what the output includes. For example common lisp compilers that aren't under a permissive license are a minefield because regardless of what anyone might say the image that gets output includes (approximately) the full language implementation verbatim in addition to the user's program.