Because, Steve, I know of no other uses of the phrase, C18 or other. And I don't believe you or Dick do either.

Many many refs in verse, and lit in general, to dew, C18 and before & after, up to Sidney Carter's "The youth of the heart and the dew in the morning"; a commonplace image for ephemerality. Probably fewer, but certainly some to fog - notably Bleak House in C19. But, as the common combo which you seem to be urging, can think of none but this one under question.

Can you?

Till you cite me one, I see no reason for the disputative, contentious [and perhaps a bit patronising] tone of your "why can't you just..."

I can't because the phrase just was not so used with the regularity implied by the tone of exasperation evident in your question.

~M~