There's a reason why an interviewer might not say what they were looking for: when a candidate raises them unprompted, that gives more confidence that they have actually done the job, rather than just memorized some bullet points.
It's a hard thing to do right though. Looking for some very specific topic may not occur to the interviewee that it's what you're interested in, so it's better to just try to give them more general opportunities to show their thinking. The ideal is to get them into the frame of mind where they feel they can reminisce about some difficult task, rather than just "answer the exam question". You get more signal that way, but you have to be prepared to give credit for whatever important things they were able to talk about.
Interviewing is hard, honestly. It's not most interviewers main task, you can go for stretches of your career without any practice, and there are many reasons other than being an arsehole that people might not do it well. And when you do it, you have a fixed amount of time, and can't easily "try another way". As a job seeker one has to realize that a largish fraction of rejections are because an interviewer "couldn't get it to work in 1 hour", not because of you. Although frustrating, as it increases the amount of time you are without a job, it is liberating in a way to realize that interviewers are just people muddling through, not perfect judges whose rejection you need to take as an accurate prediction of how others will see you.