While I think there’s something to the comments about CT-smugness as a factor, I think there’s more to it.
The reason the fears are vague, I think, is because the specifics of the fear are unimportant. What is important to the scaremongers–even the ones who aren’t deliberately and cynically using it to increase their political power–is being afraid. Or rather, making sure other people are afraid. The motive is fundamental: a desire for cohesion. One of the strongest ways to bind people together is to make them afraid of what’s outside. “It’s dark out there! There could be wolves/monsters/holes to fall into! Stay here by the fire with us, where it’s safe.”
The world is changing. In fact, I think most people underestimate the scope of the changes. That can be scary, and some people crave the basic tribal comfort of being part of a cohesive group. They want the feeling of safety in numbers. But what if the people in their group change, too? What if they become part of the scary Outside? How could they feel safe then? (This doesn’t just apply to conservatives; change can be scary for anyone. However, conservatives are probably more vulnerable to this sort of thinking because of their tendency to resist change.)
So, they claim that there are scary things out there in the dark. They don’t have to define the scary things, and they can make them up if they have to. The important thing is making sure everyone is afraid to venture too far from the fire.