And yet, that does not answer the question "How exactly does blocking car owners from working on their cars stop someone from exploiting them on a massive scale

And yet, that does not answer the question "How exactly does blocking car owners from working on their cars stop someone from exploiting them on a massive scale?" If the OTA is buggy, it can be exploited on a massive scale regardless. If you have a hardware switch that needs to be pressed while updating software, there is no possibility for massive scale exploit.