Hard to say. Sometimes Apple's OS is just legitimately missing the feature (I'm reminded of the tricks needed to have one process render into another process's window before QuickTime needed that feature for 10.6).
Honestly, I'm surprised more people aren't responding "Wait, why _isn't_ this possible in iOS? Windows has had the necessary tools to do this for years."
> Honestly, I'm surprised more people aren't responding "Wait, why _isn't_ this possible in iOS? Windows has had the necessary tools to do this for years."
Not enough people care. On the grand scale the number is tiny. I'd imagine that's because so few people have heard of this. As you said Windows has had it for a very long time but the vast VAST majority of people dnot use it, it's still niche.
When enough people want some (say user swappable keyboards, or Notification Center) Apple adds it once they have a good design.
