This is often said to be done in the name of simplicity (the 'user' is treated as an inept, lazy moron), but I think an additional, more surreptitious reason, is to keep profit margins high.

There's also one much more important reason. To quote A. Whitehead,

Civilization advances by extending the number of important operations which we can perform without thinking about them. Operations of thought are like cavalry charges in a battle — they are strictly limited in number, they require fresh horses, and must only be made at decisive moments.

Humans (right now) just don't have enough cognitive power to understand every technology in detail. If not for the black boxes, one couldn't get anything done today.

The real issue is, whether we're willing to peek inside the box when it misbehaves.