alt Hacker News> because electronic/computer aspects are mind-bogglingly complicated
And because it's software, it happens to be a perfect way for the manufacturer to extract rent (er, "recurring revenue") from car repair business. It's not complexity that's shaping how end-user repair experience looks like, but the fact that you often need proprietary connector, proprietary software, and a valid license key to interface with the car's computer.
And because plenty of engineering goes into designing subsystems with the explicit but unstated purpose of making them close to impossible to repair without ultimately resorting to help from the manufacturer.
Software is just the latest layer on the cake. Non-repairable designs, special tools, unavailable parts, unavailable instructions, fragile and error prone procedures, encryption, and more. They're all occasionally used to with the main purpose of blocking any attempt to easily repair without generating revenue for the manufacturer and their network.
Source: I have family working for two large car manufacturers both in engineering and management, who have personally experienced explicit demands to make things hard to repair by the owner but make them in a way where a reasonable explanation can be used for plausible deniability.