Ya feel me?
There are a lot of reasons not to reveal what you are working on exactly, but giving the industry isn't necessarily the best thing to do either. A lot of experienced founders will also do this because they don't want wild rumors flying around about what they are building / working on next from a one-liner that they mentioned without wanting or being able to get into the details about it with some stranger.
I have other friends who are also working at "stealth" startups who don't want to tip their hands specifically because they're backed by relatively famous people in the space. If they start talking they're going to get a lot of unnecessary and unmerited attention just because of _who_ they are and not _what_ they're going to build.
May I propose a contrarian narrative?
I see experienced founders use the "stealth mode" veil specifically because they are focusing on execution and don't really want to waste time in polite conversation debating the merits of an idea or rehashing a list of competitors. Talking too much about ideas is deleterious to the focus needed for building products. Sometimes the brain needs a break and a person wants to connect with other human beings for something other than work.
I'm sorry you weren't able to judge the last 5 people based on their ideas! You found another way, bravo!