Because we look normal for the most part.

Hi Private,
Those sorts of things have been assumed about me, too, and it hurts. . . My brother says he is tired of my "self-pity" . .
Peace and love.
Cool Cat. . .

Because they are biased and/or they feel superior? . . .

They can't understand why we should ever have depression, simply because we seem to have more than what other people are able to have, so we should never be depressed

In my case, it is assumed I am lazy and "passing the buck."