Actually not really all cars have one standard. Most German cars use Flexray which Openpilot does not currently support and is a bit more tricky to hack into as described by Comma.ai:
https://medium.com/@comma_ai/hacking-an-audi-performing-a-ma...
Only very recent cars use Flexray, and also not true for all manufacturers. CAN is still widely used, and some want to skip Flexray entirely for the next thing...
Only the really basic data (mostly related to emissions) and connector is standarised. Even the pinout of the connector (apart from the power supply and ground) differs quite a bit between manufacturers. The level of integration needed for actually driving the car is far from a standard. Not even that, most of the new cars have specific gateways which firewall the car electronics from the OBD port.
TL;DR: Apart from really basic data and physical connector, everything else is (ever more increasing) walled garden. With new VWs and Fiats requiring online connection for even most basic operations - so even if you know the protocol, you have to unlock it online (See VW SFD and Fiat SGW)
To be clear, I don't have a car or a driver's licence, I was just surprised that this standardization was a thing at all. Seems like a stark contrast to all the walled gardens in every other technology-related industry these days.