I do not think this is a lot of work. I am not an iOS developer, however I am a designer and a front-end web developer, and I think this feature, at this current point in time, is not much more than some copying and pasting of code, and a few minor adjustments. Also, we are talking about Apple, Inc. and i do not think resources would be an issue even if this was a very difficult task. I think your "resources" argument is incorrect.
The problem there is that ignores overhead of work: testing, documentation, prioritization and the like. So the work may not be tough, but larger businesses are surprisingly methodical about how they go about things. So there is more work involved that smaller businesses sometimes ignore, and that is in part where their agility comes from. But that's really beside the main point...
You dismiss my point about resources, but you fail to give a good reason beyond "This is Apple". So what if it is Apple? Microsoft
dwarfs Apple in number of developers, but even they have to prioritize their work because the number of ideas is larger than the time allotted for any one release. And then you have feedback coming from customers that can change the direction you might have for the next release and push features like this down the totem pole again.
The idea of trading in a feature for this new feature is just silly. I also think the feature is coming in the future, possibly the near future. If not for the full size iPad, the mini could especially use full screen.
It's only silly if you reject the notion that time and people are both finite resources. iOS 7 at best is looking at ~9 months of feature work, ~3 months of betas and bugfixing. Probably a bit more bugfixing time (and less feature time) if you count the time it took to stabilize the code enough for beta 1. So to fit it in iOS 7, some other minor feature would have not been implemented in its place. For all we know, they traded that feature for the time to make the iPhone version work the way they wanted it to work.
To put the feature in
now would be trading off bugfixing time for it. When you are this late in the game, you don't make that kind of trade. Not with an OS.
And I'm not trying to dismiss the merit of the feature. I'm mostly saying that there's a logical reason we haven't seen it yet: Apple simply hasn't prioritized it high enough compared to the other features they are doing, including new ones based on feedback from customers.