Modern GUI apps tend not to design for the keyboard-wielding power user.

TUIs can have just as many quirks and bugs, but on the whole they tend to be a little simpler, and the author usually is an end-user :-)

Someone is responsible for developing them.

Exactly. POS terminals are optimised for throughput. Otherwise it's literally less sales, your bottom line.

You can build a perfectly snappy, keyboard-driven GUI application; a terminal emulator is (ironically) the perfect example.