You have a good point, which is that cars have a horrible UI. One of the most important requirements is the ability to brake fast in an emergency, yet the UI requires you to physically move your foot to another pedal before you can even start to brake.

Why don't they change the UI, then? Mostly because of regulations, I think. It's unclear to me whether bringing a new driving UI to the market would be horribly difficult, costly and risky, or just plain impossible.

While you and a million others might not like the new $webapp UI, it's still a good thing that they can and do get changed. Especially in this case considering you can start using any email client anytime if you don't like the web UI.