The comparison is blatantly unfair, though: the first two examples are iPhone apps that require large touch targets, while Soulver, an OS X app, requires none. A Soulver-like app for iPhone would benefit from history and extended functionality, but for optimal usability, it would still need a screen mostly full of buttons.
The normal on-screen keyboard (or physical keyboard for those few phones that have one any more) works just fine for text input to an app like soulver. There's no need for huge dedicated buttons when the normal text input works just fine.
