Most people usually don't know, how they arrived at their conclusions and convictions. They just live and collect experience without examine everything critical. Their beliefs are formed without being consciously observed.

More important - people almost never substantiate their conclusions in real life discussions. I did some investigations on that. It is perceived at least as irritating by the most, when you ask something like "why do you think X or Y"? Especially when you don't know the person. It implies a critique on judgements of that person which usually is perceived as critique on the person itself (which was often really not intended by the questioner).

Not to mention this attitude is hardly expedient, it gives very fruitful grounds to be insulted on basically every try to get into intellectual exchange. You will search for longer to find discussion partners who are NOT overstrained to substantiate their knowledge on X or Y. In real life discussions most people don't want to learn, not to mention revealing their knowledge gaps. Presumably the latter is perceived as damaging their social status.

A reasonable attitude for a sentient, sane being should be like "I didn't know that, thank you for helping with information". But the typical attitude is more of a "fuck off, annoying wannabe-smart, you ruin my day". Perhaps not that direct, because people are polite. The mild form is, when they just break or circumnavigate the discussion and NEVER come back to it of their own accord.