Linux is open source and is mostly C code. You cannot run C code directly, you have to compile it and produce binaries. But it's the C code, not binary form, where the collaboration happens.
With LLMs, weights are the binary code: it's how you run the model. But to be able to train the model from scratch, or to collaborate on new approaches, you have to operate at a the level of architecture, methods, and training data sets. They are the source code.