The problem is that this is not the way the OEMs write the software. They are typically generated from some ecu/car/network system description using a code gene

The problem is that this is not the way the OEMs write the software. They are typically generated from some ecu/car/network system description using a code generator. So you get out this giant blob of optimized code which is essentially impossible to maintain, not to mention literally impossible to push to the upstream. On top of that, there are 10-30 ECU's in modern cars (often of 3-4 different SKUs), all with similar, but distinct code blobs. You would need a complete description of the cars network topology, the entire toolchain used to build the software and deploy it, the extremely expensive debug HW to debug it, and so on. The open source community does not develop software the same way OEMs develop software