It's not the "inference code", its the code that specifies the architecture of the model and loads the model. The "inference code" is mostly the model, and the model is not legible to a human reader.

Maybe someday open source models will be possible, but we will need much better interpretability tools so we can generate the source code from the model. In most software projects you write the source as a specification that is then used by the computer to implement the software, but in this case the process is reversed.