We know they can make them in this version of the sims because Sims 4: Cats and Dogs has an animal graveyard with animal ghosts. Unfortunately, the gaming industry is really bad when it comes to user experience vs. profit. My best guess is that the Sims 4 team is probably given a big list of things to do in a short time period, which would explain why some things in the game feel half done or don't work. I wouldn't be surprised if a different group within EA came up with the ideas for the games without consulting the game devs about implementation or user experience honestly. I could also be waaay overthinking here but it is well known that the gaming industry is awful to its employees.