It's all contracts. My father is named because the effects company he worked for had a contract for credit, the guilds similarly enforce their contracts on producers. It was common for many many people involved in production directly to not get credit, you may be underestimating the sheer number of people involved in making a major film. Industry people joke that the caterers are very important, and to be listed above them is a privilege, but a wry hint of truth because an army moves on its stomach.