I don't think the tendency is innately human; it's socialized. Growing up, it took me a while to figure out that saying something besides "I don't know" to a question you don't have the answer for, was even an option worth considering. Was never a problem for me (for better or for worse)
It's usually obvious when someone gives an answer out of their ass. Alternatively, some people will stay silent, as if they didn't hear the question. Rare to find someone who will say "I don't know" and explore the topic with you.
It's not obvious to everyone that other people are talking out of their ass. In particular, those who grow up around people who are always bullshitting, and do plenty of the bullshitting themselves, may end up not really knowing the difference.
I agree -- it's definitely not innate. My kids -- and in my somewhat limited experience, other young children -- will ask "what's that?" when you start talking about something that they don't know. It takes a few years of school to dull the natural curiosity.
It's usually obvious when someone gives an answer out of their ass. Alternatively, some people will stay silent, as if they didn't hear the question. Rare to find someone who will say "I don't know" and explore the topic with you.