If we’re talking about Apple, that is quite simply entirely wrong. The design patterns for iPhone are distinct from iPad are distinct from Mac. Hidden scrollbars just happen to solve the same basic problem on every platform – they’re elements that distract from content when not in use. It has nothing to do with saving pixels.
But if you really miss having a ui element that’s useless when it’s not actively being used just hanging around, you can always turn them on in system preferences. There’s that 80-20 again.
