Usually comedies are cheap but this one required blockbuster type levels of [bad] CGI.
I don’t think it required it, they just used it. Really CGI can be done cheaply, it’s the favorite tool of low budget indie movies. Vulture.com’s got a good article on it, basically Sony “thought” they were greenlighting the first in another shared universe series, they were going to Marvel their ghosts. So they gave it a superhero movie budget, and of course the way Hollywood works movies always spend all the budget. They could have made the movie for 30 or 40 million, and not had the psycho ad blitz, and had the same opening, and be on their way to a profitable movie. But they got big eyes disease.