> The sad truth is that the status quo for GUIs is absolutely terrible, and most were rushed out and not properly tested to check if they're really usable without a mouse.

This is the key point. No platform UI paradigm focuses on keyboard navigability and speed, all focus on mouse and visual effects for discoverability in a pursuit to dumb down the user experience enough to make it usable for everyone, breaking power users in the process (and increasingly also accessibility stacks).