He never got to see the films prior to writing. He just gets scripts and concept art. It'd be a teensy bit difficult to try and replicate an "atmosphere" of a film that's still being shot. And the stuff he wrote to develop the background on the convicts on Fiorina got cut by the publisher/ studio, like the swearing was similarly cut in Aliens.
Movie novelisations are never going to be classified as masterpieces. It's merch for the fans.