I can relate to your frustration. I sometimes want to know the source for an answer. Often the answer does give a source. If not, it's worth looking in the comments as someone else might have asked the same question. And if not you can ask yourself in the comments and perhaps get an answer from an expert there to your comment question, either the author of the answer or someone else - or start a discussion there.

I think myself that if someone answers a question, even if they are expert, that it's good to give a citation if they can. Because - it's surprising how often you think something is true, but when you check the citation, then you find that you missed out some important small detail that you'd forgotten, or didn't realize was so important, and that can slant your answer and even make it incorrect.

It depends. I mean if it is a question about airplane pilots and the answer is by an airplane pilot, say, about the layout of the cockpit and the instruments they use and protocols they follow, it's absurd to ask for a citation. That's where quora scores over wikipedia. On wikipedia an airplane pilot could not just describe the cockpit layout and explain what every instrument does, they also have to find citations to articles online describing how airplane cockpits work. Which may seem absurd, though understandable given the way wikipedia works, if they fly that plane every day themselves.

When I answer questions then I give citations as much as possible. Yet still often get asked for citations on things I said that I forgot to give a source for, or assumed that "everybody knows the source".

Then another factor, on Quora experts or indeed anyone can write opinion pieces where they give their own conclusions, and the reasoning they themselves used to lead to that conclusion. So long as that is made clear, that you don't attribute it to someone else falsely, I think that's good also. In that case then the author themselves is the source. They might sometimes give links to other things they have written on the same topic. Again I see this as a strength of quora - combined with the possibility for many people to write their own opinion pieces as answers to the same question. Many questions are even phrased in such a way that it is clear the author is asking for your personal opinion.

Some answers have comments switched off. Though that is an option in quora and they are doing nothing wrong to do that, I think it is generally not a good sign. Those answers are often somewhat lower quality and for me it gives the impression that the author is so sure they are right they aren't interested in any corrections or comments - which usually, paradoxically, means they are wrong or at least not correct in every detail.