A common comment, but rarely true.
Every day we hear someone say, "that's interesting" — or we say it ourselves. But it's rarely true.
Something that's truly interesting is probably ponderable or worthy of some thought; so the quick comment about something is just a kiss-off…a reflection of not knowing what else to say.
Simple curiosity about something probably doesn't qualify it as interesting. But "that's interesting" tends to get us out of an awkward conversation.