Important ideas don't always require long explanations. Here's a famous example:

Let an ultraintelligent machine be defined as a machine that can far surpass all the intellectual activities of any man however clever. Since the design of machines is one of these intellectual activities, an ultraintelligent machine could design even better machines; there would then unquestionably be an “intelligence explosion,” and the intelligence of man would be left far behind… Thus the first ultraintelligent machine is the last invention that man need ever make.

This is a single paragraph on the third page of a 50 page report. Maybe someone who's good at parsing 60s era academic English can tell us if the rest is any good.

It seems like anyone who has an idea they want people to take seriously has to write a bunch about it. This is most apparent in popular nonfiction books, which are often bloated far beyond what it takes to communicate the core ideas.

To correct this "presentation length bias", we can fight it from both ends:

Remember that important ideas don't have to be in an important place, be said by an important person, or be an important length.
Alert readers to important ideas that don't look important (e.g. "This is a simple idea, but it seems important:"). Do this especially if it's someone else's idea, since people are going to be reluctant to label their own ideas as important.

If we get rid of this bias, the biggest win may be that people work harder to present important ideas concisely, since it won't cost them prestige anymore.

See also: Why do we write too much?