Think about it this way- in BI, the factions are far less balanced. Rome is huge, which either means that it's easy to steal provinces from or a province eating monster depending on the difficulty level. It wouldn't make sense to have an AI Rome victorious just by conquering more of sarmatia and taking the other capital of the empire, because we all know it would've taken alot more than that. Making it impossible for Rome to win if you're playing as a faction other than Rome makes some sense. Also for hoard factions it may take awhile to get wherever you intend to establish a new homeland, and to have all the other factions working toward their goals for 50 turns before you get to start yours would be irritating.

Even still I agree it removes some of the fun.