Have you ever thought about how in shows like Star Trek almost all of the species are roughly human sized? You never see a race where everyone is 14 foot tall or maybe only a foot or so tall.
Even when shows don't have the technical limitation of having to film with live actors on the same set they still usually stick with that with the occasional exception. I guess it makes composing scenes easier. There's no way I could work in Lego Technic guy here if he wasn't sitting down.