Davis and Baldwin can't be in the third for the same reason they weren't in the second. They've aged. At the end of the first movie they were ghosts. Ghosts don't age. Davis is clearly much older now and Baldwin could lose enough wait fast enough to play the same character let alone the Charme in his face.

Keaton is covered in makeup and his face hasn't changed all that much at all. The remaining Deets family are all still living so their aging is just natural.