Since the times when your handle on Twitter was your username, when some guy named Tom sold the handle @Tom to someone else for hundreds of thousands of dollars, this became common practice. Whenever new social media appears, some people will go in and create all the communities they think have a chance to be used someday. Communities for cities, possible new games, companies etc. Then when the time comes, they either sell the domain to the one who wants it, or they become the admin of a community without working to build it up. That’s how reddit power mods became a thing.