Knowitallism? We are all armchair psychologists (see, there I go making assumptions about other people).
Maybe it comes down stereotyping, and the ease of making assumptions. "Of course you are like <insert stereotype> because popular media/something I read/a movie made common sense to me, and therefore it applies to ALL". Challenging people's assumptions are hard. Heck, it's taken me years to get past some of my own inbuilt assumptions, and I LITERALLY KNOW facts that go against my assumptions yet I fall back into them.
There's also something I've seen other people do. Where they interact with another person, that person leaves. Then in private to they will say, "Well, we really know what's going on there, don't we?", with a knowing wink, nod, or other non-verbal clue. Heck, I've probably done that from time to time.