That's not true at all actually.

These sort of dialogs happen all the time:

Consider the following - "I know Google stocks will be over $700 by the end of next year." The natural and most common response is probably, "You can't know that for sure." Consider the following - "I know that all dinosaurs had feathers." The natural response is, "That isn't necessarily true." That's why typically these things are phrased, "I predict Google stocks will be over $700" or "I believe all dinosaurs may have had feathers".

The uncertainty is implied, claiming certainty is still false. There are reasons why the terms, "probably", "most likely", "appears to be", and "I think" or "I believe" are so common. These are the phrases you use when you talk about something that is not certain, that's why epistemology exists - not as some Matrix conjecture, its the basis for the fundamental tenets of knowledge - namely that you need to understand what you know and what you do not know.

If you're going to question why I'm not certain about something, you should know whether or not what you assume I should be certain about has any certainty to it.