All you have to do is look at the use cases of a tablet. They are designed, in Apple's implementation, to be immersive. The screen is dedicated to a single app view that may have multiple sub views. This is iOS developers guide 101.
While there are gains in performance to be had on multiple cores, you start getting limited after 2 due to the single threaded nature of most processing tasks. So yes, you may have hundreds of theads running but many of those are queued up and are serial in nature. In short, you quickly reach a limit on how much a tablet or phone benefits from multiple cores. Most people hit that limit at 3-4 cores on a desktop. Tablets, I am betting, are around 2.
So if you have limited silicon to work with, where do you put your resources? Given the Retina screens, the GPU is the first obvious choice. Pushing pixels is a highly parallel task and doubling the performance of the GPU doubles the performance of times when pixels are being moved; that happens all the time. From games to word processing, iOS benefits directly from faster GPUs.
The other option is to increase clock rate and increase single thread performance. ARM, really lags traditional designs, like the Intel Core micro architecture, in this area. To get there you use smaller topographies (allowing higher clock rates at the same power usage) and updated ARM designs like the Cortex A15.
In the future, like iOS 8.0 or 9.0, the design paradigm may shift to a multi windowed touch iOS but that is still a few years off and hardware should be optimized for the current use cases and not some far off future use case that may never happen.
You keep saying that, but haven't substantiated it with anything but your opinion.
That's my point. You don't have any evidence to support your claim that a tablet can't use more than 2 cores. Please explain why Apple put 4 graphics cores in the latest iPad, for starters.
(Yes, I know that graphics is different than CPU, but there's no reason that the OS and apps couldn't be multithreaded enough to use multiple CPU cores, as well as graphics cores).