A CLI is an interface that lets the user communicate using the the computer's language.
Siri is an interaface that [attempts] to let the user communicate using their own language.
It's a fundamental difference I think. Indeed the only thing that Siri and CLIs have is that they are non-graphical.
This is actually the similarity that matters. In a GUI interface, you have to page through "commands" to find what you are looking for. In CLIs and Siri, you simply go straight to the command by typing or speaking.
Programmers love CLIs because, after the initial cost of learning the commands, you can do things much faster than a GUI. But, speed of executing commands is less important on the desktop than on a phone, which is why I suspect most normal users don't bother to learn a CLIs.
But when you're on a phone with trying to tell your boss you're running late while coming down a flight of stairs, Siri is going to come in real handy.
They've been satisfied enough with the GUI because the alternative, the command line, was several orders of magnitude harder to learn than the GUI. Siri is comparable, or possibly easier, in terms of learning difficulty.
Fact is, with a command line you don't "talk" to a computer. You actually program it
But with Siri, you talk to it. That's where the NLP piece comes in. "Wake me up at 7am" works, so does "Can you set an alarm for 7am?", etc. The syntax is deliberately loose and not programming-language-like. So you don't need to be exact at all.