Ok, let's consider your example. Japan (in the hands of a skilled player, due to game's imperfect historical accuracy or thanks to pure luck) steamrolls China. They then take over South-East Asia from the British and invade India. Would it still be "historical" and allowed in such a game? If yes, Japan, UK and India should surely have ways to react to it leading to more and more deviating from history. Anyway, you'd end up with something pretty weird such as Japan attacking Iraq in 1945 or India surrendering and joining the Axis etc. We'd still have lots of complaints from history lovers that it can't happen in a historical game.
And if you disallow such a thing, what's the point in winning the war if you can't exploit the spoils of victory? Again, it would mean the game would turn into a series of tactical battles with strategy completely in the hands of developers. I don't mind this genre, but it would be a very different game from HOI4.