Modding an existing game is rarely comparable to the complexity of creating the game from scratch.
Adding assets and changing dialigue in and existing dialoge framework are accessible to a lot of people who are not up to designing physics and hitboxes. AAA titles require vast teams of people to get the skeleton and muscles of the game right, and mods are generally the hair coloring and nail polish.
That isn't to say creating mods is inherently easy or anything, but it is tweaking an existing thing and not comparable to building it from scratch.