Because we know what LLM's do. We know how they produce output. It's just good enough at mimicking human text/speech that people are mystified and stupified by it. But I disagree that "reasoning" is so poorly defined that we're unable to say an LLM doesn't do it. It doesn't need to be a perfect or complete definition. Where there is fuzziness and uncertainty is with humans. We still don't really know how the human brain works, how human consciousness and cognition works. But we can pretty confidently say that an LLM does not reason or think.

Now if it quacks like a duck in 95% of cases, who cares if it's not really a duck? But Google still claims that water isn't frozen at 32 degrees Fahrenheit, so I don't think we're there yet.