Do we need a realistic simulation at all? I was thinking about how educational games could devolve into, instead of "guessing the teacher's password", "guessing the model of the game"... but is this a bad thing?

Sure, games about physics should be able to present a reasonably accurate model so that if you understand their model, you end up knowing something about physics... but with history:

actually, what's the goal of studying history?

• if the goal is to do well on tests, we already have a nice model for that, under the name of Anki. Of course, this doesn't make things really fun, but still.
• if we want to make students remember what happened and approximately why (that is, "should be able to write an essay about it"), we can make up an arbitrary, dumb and scripted thing, not even close to a real model, but exhibiting some mechanics that cover the actual reasons. (e.g. if one of the causes would have been "not enough well-trained soldiers", then make "Level 8 Advanced Phalanx" the thing to build if you want to survive the next wave of attacks.)
• if we'd like to see students discover general ideas throughout history, maybe build a game with the same mechanics across multiple levels? (and they also don't need to be really accurate or realistic.)
• and finally, if we want to train historians who could come up with new theories, or replacement emperors to be sent back in time to fix Rome... well, for that we would need a much better model indeed. Which we are unlikely to end up with. But do we need this level in most of the cases?

TL;DR by creating games with wildly unrealistic but textbook-accurate mechanics we are unlikely to train good emperors, but at least students would understand textbook things much more than the current "study, exam, forget" level.