The problem is the response to that is almost always "Oh you mean like ______?". The answer to that is almost certainly no, but you can't really say or explain why unless you reveal too much about your startup at too early a stage. Then people get the impression you are working on some stupid idea that has already been done.
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.