Exactly. It's not like these people couldn't have dreams where a dinosaur stampede suddenly happens, or they all start flying. It's not like the city couldn't just fold in on itself. It's just that not everyone has flying dreams. Not everyone has dinosaur dreams, or if they do, dreams about dinosaurs the same way. My dinosaurs are always mean and eating my family and friends. Other people may have a pet dinosaur that they ride around Saturn's rings or something like that.
On the other hand, everyone understands their world, and probably has mundane dreams about it on occasion. By making the dream as close to something in the target's world as possible, you negate the risk of throwing things at them they'd never dream of themselves, thus alerting their subconscious that there's an impostor in the dream.