We do. The iPhone and iPad have a single physical button. This forces the OS to have a flexible, on-screen keyboard which changes state depending on context and utility, effectively advancing the state of text input.

On top of that, most phones now have excellent voice input, so you can skip the keyboard altogether.

All of this because we made devices without keyboards at all. Necessity is the mother of invention.

Because it's all about trade-offs, and a one-button keyboard is a ridiculous trade-off.