Well, I found out why the Maitlands weren't in this

Gina Davis thought she would look too old for a comeback role (and it makes sense, because Barbara and Adam aren't supposed to have changed at all in 37 years, since they're ghosts). Plus, an even bigger problem would have been Alec Baldwin. He apparently got caught in a few scandals of the underage kind, so he's persona non grata in Hollywood right now. Not to mention he killed someone on the set of his "Rust" movie while filming.

It's too bad. I know how they could have managed it, if the writers had been serious about including the Maitlands. In the unlikely scenario the two actors could have been brought on, they could do voice acting for the scenes, with body acting done by two actors in costume and wigs, who had the exact height and body types Gina and Alec had back in 1988, and AI could have been used to put their young faces on the body doubles in post-production. Their lines could have been done by both the original actors, and the body doubles, and then Gina and Alec's voices could be dubbed over [again] in post-production.

Sadly, that would have cost a lot to do, and the movie was probably already having to stay on budget for special-effects anyway. However, the audience probably would have liked seeing those two again, and probably would have been briefly wowed by how "young" Gina and Alec would have still looked, preserved as 1980s ghosts.