Great answer from Quora:
Three reasons. They look scary because they’re unfamiliar; they look scary because they’re not accurate reconstructions; and they look scary because they were, in fact, scary.
The third part is pretty simple. A multi-ton carnivore is a scary thing, and it would be scary if it was wearing big clown shoes and a red rubber beepy nose. Although there were lots of small dinosaurs, there really were a lot of huge ones, and things that are that much bigger than us are dangerous.
The first point is also fairly clear, I think. Dinosaurs used different solutions to many problems than the animals we’re used to seeing, especially big animals, and we are jolted by those differences.
Finally, the reconstructions we see tend to be biased toward scariness. Dinosaur reconstructions tend to be done conservatively, meaning that muscle mass is minimized and the skin is stretched tightly over that mass. Surface adornments and colors are not known, so they’re not added.
Check out the photos to boot over here. Interesting stuff.