I was going to say that at least tablet users who want keyboards at least buy them as an add-on rather than buying tablets with keyboards built in. Then I realized that a tablet with a built-in keyboard is called a "notebook computer", and they're already quite popular.

Seems to me that it's all about the difference in size. A full-sized physical keyboard is well superior to a virtual one. But on a tiny phone, either way is going to be painful, so it matters much less.