Often a trailer and bullshots are created to sell a project to the publisher. This happens before the game engine architecture and art pipeline are ready to use. So it's easier and quicker to render CGI scenes. Besides making the funding possible, a trailer also gives the team an idea of what they're collectively shooting…
Most of the time the game is up and running by the time we're contracted for a CG trailer. However we are nowadays asked to do stuff that's not really possible in engine for some reason - extreme close-ups, effects, lighting, scene complexity, and so on. Also, games are still quite behind in dynamics simulations for hair,…