I think fear probably plays a significant role on both ends.

On the consumer side of thing, there is a fear of loss. A lot of people don't want to even mess with their own oil, headlights, etc. for fear of messing up their cars. A lot of people don't have the luxury of extra cash lying around to pay to fix any of their mistakes nor a second car to tool around in when their first car gets messed up. As a result, they opt for better safe than sorry.

I know similarly, for computers, my first foray into Linux was on a busted ass computer that was so janky, even if I did fuck something up, I wasn't at a loss because the computer was practically a paperweight as it was. I was already looking at buying another computer dead in the face so I didn't really have anything to lose at that point. It wasn't until I had nothing to be afraid of that I tried to do something new.

As for designers and engineers, I think they're afraid that if they make their products too easy to work with, they're risking their company's assets. "Hey ho. If we make this cell phone easy to jail break, Apple could lose revenue and I could lose my job for not making a more locked down system. Better safe than sorry, let's put this shit behind a brick wall."