I would think, given my travel and cultural experience, that the US would be one of the least problematic countries from the PoV of being allowed to challenge "authority" and ask "why" without catastrophic career consequences, but would be interested to hear other perspectives.
I never found my co-workers to be defensive when I asked them the same question in Japan (in Japanese). I didn't have any subordinates, and management let the group decide, (at our level), so challenging management rarely came up.
I agree that asking "why" is probably not a career-ending move in the US -- I'm just talking about the subtle first-response I've notices from other Americans in the first place. I don't know that it has any larger relevance than to maybe lend support to the idea that US workers (or engineers, anyway) are afraid to be wrong in public too.
But, in companies that manufactured products intended to compete in the global marketplace (and not something intended to meet some local need), the internal processes for doing root-cause analysis of design, manufacturing or even business model defects was pretty direct/efficient/ruthless and impersonal. To me that makes some sense: If I were on a team of 100 NASA engineers, all of us armed with slide-rules, trying to send some people to the moon and back, I wouldn't want feelings (or social standing) to get in the way of the process of verifying that the calculations were correct.
Your two questions could elicit a better response, even though it's basically the same as "why," because it's at least showing a semblance of effort and the interest to probe deeper.
It's an important information-seeking question!
But, for some reason, in many geographies, here, in the US, people often (apparently) take it to mean something like "I don't agree." or "Justify that.". And that triggers defensiveness, and then you aren't getting information.
Because my goal is the information nugget at the end of the quest, I've learned, with most people, until I know them well, to use something like these instead of "why?":
I don't know if that makes Americans thin-skinned, or just sensitive to that one word, or ... what ... but I think that most Americans most of the time would prefer to not offend, and have learned that challenging another person's opinion is viewed by many to be offensive. It's highly inefficient, doesn't work well in multi-lingual meeting settings, so I'd guess the Americans you have been exposed to are are somewhat experienced in international business?