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)