While writing might be a nice additional feature, I can certainly understand why Apple didn’t build iOS around it. (In the current climate this probably also means that handwriting won’t come to iOS anytime soon.)
I think we can thank RIM for that. While they were not the first, they arguably made the QWERTY keyboard ubiquitous and (for the first time) more than tolerable on a phone sized device.
That's not to say it's the optimal method of input, but either is voice. There are use cases where writing or voice may be better than typing.