Because 99% of the time it's not what users want.

You can get it to ask you clarifying questions just by telling it to. And then you usually just get a bunch of questions asking you to clarify things that are entirely obvious, and it quickly turns into a waste of time.

The only time I find that approach helpful is when I'm asking it to produce a function from a complicated English description I give it where I have a hunch that there are some edge cases that I haven't specified that will turn out to be important. And it might give me a list of five or eight questions back that force me to think more deeply, and wind up being important decisions that ensure the code is more correct for my purposes.

But honestly that's pretty rare. So I tell it to do that in those cases, but I wouldn't want it as a default. Especially because, even in the complex cases like I describe, sometimes you just want to see what it outputs before trying to refine it around edge cases and hidden assumptions.