Hello,
Alot of these doomsayers are able to sell various bits of merchandise for extortionate prices, simply because they know there are many people scared and desperate enough to waste their money on things like that. Beyond money they get recognition, status, and usually a cult following made up of insane and/or deluded people.
There really isn't one reason for their behaviour, and sometimes it's difficult enough trying to find the reasoning behind their lies. The Heaven's Gate cult for example, committed mass suicide when comet Hale Bopp appeared in the sky, thinking that their souls would be transported to a supposed alien spaceship following the comet. How do you make sense out of such nonsense? You don't because it's nothing more than insanity and stupidity, and their doomsday theories are just a figment of their imagination, and nothing for you or anybody else to worry about.