Smart People

I'm sure there are a lot of people who think that those around them are dumb because they never talk about anything interesting. The truth is there are probably some people that really have nothing interesting to talk about most of the time, but I think many very smart people don't consider sharing what they know because it's so obvious to them, and instead of being something always on their mind, it becomes a part of them--so much so that they don't even consider it knowledge.

Say someone was watching you work a math problem and you multiplied 9 by 8 in your head. They ask how you got to 72 so quickly. You'd probably think, "9x8 is just 72..."
Our multiplication tables are so deeply ingrained in us that we don't really even consider it knowledge.

When someone learns about/gets better at/practices something, they lose sight of what's common knowledge. And what tends to happen is they lose the incitement to try to share with those on a different level of understanding because it's frustratingly futile.

Regard my cheesy drawing depicting competence on an artful level.

In short, the stick man on the upper level doesn't have the motive to descend to man on the lower level using the dumb elevator. He'd rather hang around and wait for someone to come up a level to talk to him.
(I suppose a set of stairs would have been more befitting to convey my point, but I already closed Microsoft Paint.)