Training is too expensive for the training data to be the preferred form for making modifications to the work. Given that, the weights themselves are the closest thing these things have to "source code".
And this is where the reproducibility argument falls apart: on what basis can we insist that the preferred form for modifying an LLM (the weights) must be reproducible to be open source but the preferred form for modifying a piece of regular software (the code) can be open sourced as is, with none of the processes used to produce the code?