Autommatic, Wordpress developers have done this because their main service wordpress.com uses the same feature. Today this feature is very popular if you are doing something more than a personal blog about cats. Nodebb has a ton of examples of how other forum services (like Reddit) were made. Maybe it will be their way too. In any case, I hope that someone will make a plugin that allows you to use one file base and easily manage many forums from one place.
@kenygamer I'm far from a developer. The folks at email@example.com are probably the best bet for a customer plugin. It sounds like you don't want to use single sign on (which relies on cookies or session tokens), you want to use a full on external database for authentication.
I think from a user experience, cost, and level of effort; you'd be better just having the users register on the forums.