In many cases, it's an ego thing... some people like to feel that they have special, important knowledge denied to ordinary mortals. The next step is to share that with said ordinary mortals, and to garner the acclaim to which said knowledge should entitle them.
In other cases, people have sincere concerns about dangers or trends that they see in present-day society, and feel that they have a duty to speak out about them. I think that this urge is present in some of the dystopian movies you mention. And sometimes, this urge can be quite helpful: for example, the ozone depletion problem, discovered in the late '80's, or the Y2K hysteria of the pre-millennial years.
In both those cases, a serious problem was identified, became part of the popular culture, but was solved by sensible measures carried out with a lot of hard work. (The Montreal Protocol banning the use of the most dangerous ozone-depleting chemicals in the first case, and the widespread 'patching' of old software in the second.) And in both cases, there was a popular 'hyping' or 'spinning' of the problem as an adjunct to the more serious consideration of the problem. That's particularly true of the Y2K example.
Finally, in both cases, revisionist writers--or writers who haven't adequately investigated the history--have claimed that the 'whole thing was hype.' Apparently all they recall (or care to acknowledge?) was that there was a Doomsday-like fear, and that in the end nothing so terrible happened.