I think it very much depends on the how (as often). Very different to ask something along the lines of "Why didn't you just do X?" (the unwritten "duh!" being

I think it very much depends on the how (as often). Very different to ask something along the lines of "Why didn't you just do X?" (the unwritten "duh!" being very implicitly present!) vs. "Interesting. Naively I would've have thought that doing X might solve this and seeing you didn't go with that solution I'd be interested to understand the constraints and reasons that make X unviable." With the first (type of) question you've disqualified yourself already, even though you didn't 'just give the solution' (which is even worse). After that kind of question we're already on adversarial grounds. The "5 whys" is a great technique but if literally just ask "Why" a lot, you'll get the boot more often than not ;)