That was not always the case. Sticking with the Epic example, a great deal of custom maps made it into regular rotations in the original Unreal Tournament comm

That was not always the case. Sticking with the Epic example, a great deal of custom maps made it into regular rotations in the original Unreal Tournament community, and a great many mods were played for a very long time. But people weren't doing it for money, or "brands" as you say. They were building stuff for their own enjoyment. A community polices itself, ultimately. If "brands" try to police it, then yes you will get penises and Hitler, because "brands" have assumed liability for said policing and the community itself is rightfully reluctant to help them do their jobs. Penises and Hitler are more often than not a subconscious lashing-out at the "brands."