The problem with a open-world type game set in dragon ball z is...well the mobs frankly make no sense at all.  Everything that's just a mob would be useless to fight since even during the saiyan saga, Goku is vastly stronger than anyone on Earth except for a few rare individuals that could put up a fight.  It makes no sense for him to fight red ribbon robots and dinosaurs as they pose literally no challenge at all plus there shouldn't even be that many of them on Earth in the first place, especially red ribbon stuff.

Now, if they set the game during Dragon Ball, then Earth mobs would actually make sense and pose a threat, especially during the earliest parts of his career and would fit well as a game mechanic and fit plot wise.  Also, during Dragon Ball the Earth seemed a lot more interesting, once it moved to Z and beyond it was basically reduced to just home base and a place to get invaded or destroyed.  Earth enemy mobs just really don't make a lot of sense considering the time frame in Z.

Once you get into Super, or even GT, then mobs could make sense again since there are a ton of planets or even Universes to visit which would open up a lot for enemy variety and types.  In a super setting, you could just make up a whole bunch of worlds and places to explore and train.  In a GT setting there were already quite a few different worlds and locations that could prove interesting to explore and interact with the locals.  But of course game developers want to stick with what everyone remembers best cause they don't like taking chances and instead just rehash the same thing over and over.