A keyboard is a mechanical thing, so it needs small bits of plastic/metal to be machined and made. That kind of manufacturing is very expensive to do and can't really get any cheaper.
The reason that computing technology gets cheaper and cheaper is that there's a huge research and setup cost to make silicon chips, but actually churning them out once done is very cheap. So the chip manufs tool up for top of the line computers and charge a ton for them. So then they find a way of making them even better but they are still tooled up for making the previous stuff so they can keep churning it out at smaller and smaller cost.. so it proliferates into more and more cheaper kit like phones, watches, ipods etc etc.
This doesn't happen with mechanical things cos tooling up is less of an issue but each item is expensive. So it's either dead expensive or really crappy. Or both 🙂 There have been fold-out keyboards for PDAs but I've neve seen anyone use them – too much trouble I suppose.
They have done a thing that projects a keyboard onto a table with lasers and you can type on it, but again it didn't seem to catch on. I suppose no-one wanted it 🙂