Yes, the issue with the portrait keyboard is coming up with something that fits 10+ characters on the same row, while still being usable.

You can in theory do a design where you zoom the key after each press, then let go when you're on top of the right key. But solutions like that (for instance in the iPhone) use an additional predictive engine to "smooth out" / guess what character the user was actually pressing. Without the predictive component the experience simply wouldn't very good.

Of course you can say that you just need to type so that you carefully select every letter there, but then the typing experience wouldn't be very satisfactory. The bar of just turning the device to landscape and typing with the nice HW keyboard gets very low - and if so, the bother of making that keyboard in the first place might get lost.

(Qgil has the additional point about one hand use: if portrait keyboard requires two hands, you might just as well use these two hands to turn into landscape.)