The disks we've been used to seeing on recent Macbooks are at least an order of magnitude faster than the storage we have in the iPhones, so that is one reason the laptops can get get away with using them. Power utilization becomes a bigger concern on phones as well, because if you implement writing to disk for more tasks, you are keeping the CPU from going idle more often that it would otherwise be able to when operating with memory instead - that memory can accomplish the same task faster allows the CPU to sleep earlier (provides savings in power, keeping temperatures closer to ideal levels, etc.). Here is a crude scenario (e.g. off the top of my head, non-optimized, etc.) - implementing disk caching on browsers causes situations such as when I was attempting to transfer Xcode to my phone; depending on how many objects are being created so that a particular site can be rendered, you are potentially running disk I/O for long periods of time, which can be detrimental to battery life. You can also run the risk of queuing sufficiently large amounts of disk I/O operations to the point of causing processes to stall (again, due to the low speeds of disk I/O).
Writing to disk would not require a re-write to iOS, as APIs exist for those already (e.g. a game app stores its save data), but enabling swap files would, unless I'm mistaken. Keep in mind that everything I've been discussing so far is purely theoretical based on my computer architecture knowledge (I work with software, not hardware), so there may exist practical methods of implementing disk-based caching functionalities without hamstringing the phone's overall experience.
