That's a fair question. What would keep me from using an iPad to write a novel? Certainly not processor power, and certainly not lack of software, because you're absolutely right, the iPad has those covered. You're also correct that you'd have to attach a proper keyboard. The small screen is a consideration, but I've managed in the past with a 14" monitor, so I wouldn't really complain about that either. So an iPad with the right peripherals would give me the technology I'd need.
But it's not about the technology. It's the setting. Wordsworth said you get poetry as "the spontaneous overflow of powerful emotions recollected in tranquility: it takes its origin from emotion recollected in tranquility." (http://en.wikipedia.org/wiki/William_Wordsworth) Now, I don't write poetry. But the basic process works the same way, even for my plain old humble science fiction and fantasy. Inspiration comes wherever and whenever it will, but when I sit down to work it into something worth reading, I need a place as close to a sensory deprivation tank as I can get. A desktop ties me to the same monotonous desk in the same monotonous room, so the environment fades into the background and I can work. There's no temptation to take the gear on the road and write in the park or by the river or wherever.
I think it would take more discipline than I have to work on an iPad and not be tempted to think I could get things done anywhere. I'd have to nail it to my desk. But if I did, I'd give you the point. It's not the technology's fault.