I work on automotive software (not Tesla), and it's like this partially because it makes development _way easier_. Rather than needing to get a whole car to the dev team, you just give them the specific part that they're working on. Anything that needs outside features usually just fails gracefully (e.g. no speedometer or no location for maps). These are usually mocked for testing, or you add the specific ECU that provides it for your testing setup if needed.

Modern cars have tens of ECUs, so if you had to have all of them for testing, that would get unwieldy extremely quickly. Not to mention that cars are pretty resilient to having random parts failing, you don't want to lose the entire dashboard just because the ECU that provides camera data failed, or something.

Yeah, I expected some gigantic writeup about tricking it into thinking all other systems are connected to it but maybe it's made this way so it's easier to repair without needing the whole car