Why Doesn't OSX root user automatically default to the same shell as other users

I know this is a stupid question, but I have googled this and searched Stack Exchange, and have not found an explanation for why the root user on OSX doesn't automatically default to the same shell as other users. Seems like this would avoid a lot of complications, but I guess there must be complications with the shells being the same for root as others, but I just don't know what they are.

Thank you.