Because making people who don’t yet have a solid grasp on reality act is kinda weird.
I was originally going to make a case for animation here. If you don’t require child actors, you can have a very large cast of kids play very complex roles at the same cost as you would with adults playing the same roles. Because, you know, child actors are a pain to deal with, and there’s certain things that are just so much easier to pull off with puppets.
However, this isn’t going to be limited to animation anymore. Remember Avatar, that Pocahontas-in-space movie? They overlaid alien CGI bodies on top of human actors. That movie came out in 2009 (and is apparently getting a sequel in 2020? eww), that’s eight years ago. These days, we should be able to fairly convincingly substitute actors for animated doppelgängers. And if you can do that for children, I’m hoping to see a surge in off-the-scales crazy movies with “kids” playing all kinds of sinister roles.
Ah, the future, when you don’t even know if the actor on screen was alive during the filming of the movie.