When you're trying to cut the BOM and weight, sharing a network can seem like a good idea.

Also industries have cultures, and dunning-kruger often applies outside their core domains.

For example: I did some work with Mercedes (no insult to them -- I've happily owned several of their cars). They were "real" engineers; the "schnook" of the door when it shut. The brake-by-wire folks modeled everything in Matlab, developed multiple implementations, But the entertainment system? It's not "real" engineering, so is typically subbed out to a canonical or cheapest bidder based on a powerpoint bullet list. The result: really really safe vehicle, but the user's actual experience (outside the driving) is not encouraging.

Same issue with a BMW I had: ECU always kept the pollution within spec; the car always started instantly when I turned the key no matter the temperature or environment, but the seats and windows would move randomly.

And we all suffer from this: when I worked on power plants I was (and remain) horrified by the shitshow that is SCADA and the HMI infrastructure many of the other "absurd" things I saw, well, some patient people would show me what an idiot I was and that there were very good reasons for doing things that looked to an outsider like headstands.